What Is a Brother-Sister Controlled Group of Companies?

A brother-sister controlled group is an IRS classification that treats two or more businesses as a single employer because the same small group of people owns enough of each one. The label turns on ownership overlap, not on whether the businesses do similar work or deal with each other. Once companies meet the definition, the IRS folds them together for retirement plan testing, health coverage obligations, and several tax deductions and credits that would otherwise apply to each company on its own.

The Ownership Test

The core rule sits in Internal Revenue Code Section 1563(a)(2). Two or more corporations are a brother-sister controlled group when five or fewer individuals, estates, or trusts own more than 50% of each corporation, counting only the identical portion of each person’s ownership across the companies being tested.¹

“Identical” is the word that catches people. The test doesn’t ask whether the owners together hold a majority of each company. It asks how much of each person’s stake overlaps across the companies. Only the overlapping portion counts, and that overlap has to exceed 50%.

The math is easier with numbers. Say two people own two companies:

Owner X: 70% of Company A, 30% of Company B
Owner Y: 30% of Company A, 70% of Company B

For each owner, take the lower of the two percentages. That’s 30% for X and 30% for Y. Add them: 60%. Because 60% is more than 50%, these two companies form a brother-sister controlled group.¹

A brother-sister arrangement is different from a parent-subsidiary controlled group, where one corporation directly owns at least 80% of another. Here, no company owns any of the other companies; the connection runs entirely through the individuals above them.

When the 80% Test Also Applies

The original definition required two things: the “more than 50% identical ownership” test and a separate “at least 80% common ownership” test. A 2004 amendment removed the 80% requirement from the core statutory definition used when apportioning tax benefits among group members. Treasury regulations kept the stricter two-test version for any law outside Sections 1561 through 1563 that references the controlled group definition, including the employee benefit rules under Sections 414(b) and 414(c).²

Under the two-test version, the same five or fewer owners must:

Collectively own at least 80% of the voting power or total value of stock of each corporation, and
Own more than 50% of each corporation when only the identical portion of each owner’s stake is counted.

So the same businesses can be a brother-sister group for tax-benefit purposes, where the 50% test stands alone, but not for retirement plan purposes, where both tests have to be met. Most structures that clear 50% also clear 80%, but the edge cases matter when they appear.

Family and Constructive Ownership

The IRS doesn’t stop at shares held directly in a person’s name. Section 1563(e) attributes stock owned by certain family members and related entities to the individual being tested. The point is to stop owners from parking shares with relatives to escape controlled group status.¹

The family rules vary by relationship:

A parent is automatically treated as owning stock held by a child under 21, and a minor child is treated as owning a parent’s stock.
For adult children (21 and older), attribution between parent and child applies only if the person being tested already owns more than 50% of the corporation.
Spouses are generally treated as owning each other’s stock. An exception applies when the non-owning spouse has no direct ownership, doesn’t participate in managing the company, the company earns less than half its income from passive sources, and no restrictions favor the individual or their minor children.
Attribution between grandparents and grandchildren applies only if the person being tested already owns more than 50%.

Stock subject to an option is treated as owned by the option holder. Stock owned by a partnership, estate, or trust can be attributed to partners or beneficiaries who hold at least a 5% interest.¹

Not Only Corporations

Section 1563 itself speaks to corporations. Section 414(c) extends controlled group treatment to all trades or businesses under common control, whether incorporated or not.³ LLCs, partnerships, and sole proprietorships can therefore form brother-sister controlled groups for employee benefit purposes. If you own an LLC and a corporation, the combination can trigger controlled group status just as readily as two corporations would.

What It Means for Retirement Plans

This is where brother-sister status creates the most trouble. When companies form a controlled group, every employee across the group is treated as working for a single employer for retirement plan purposes.³ That one rule reshapes almost everything about plan administration.

Coverage and Nondiscrimination Testing

Qualified plans like 401(k)s have to pass annual testing to show they don’t tilt benefits toward highly compensated employees. In a controlled group, the testing pool is every eligible employee across every member company. Separate tests per business aren’t allowed. A 401(k) at one member that mostly covers well-paid staff will often fail once the lower-paid employees at a sister company are counted in.⁴

This catches more owners than you’d expect. A doctor who owns a medical practice and a real estate LLC may find the rental employees have to be included in coverage testing for the practice’s 401(k).

Contribution Limits

The annual cap on total additions to a defined contribution account, $72,000 in 2026, applies across the whole controlled group rather than per company.⁵ An employee who works for two brother-sister companies can’t receive $72,000 from each. The elective deferral limit, $24,500 for 2026, likewise caps what an employee can defer across all plans maintained by group members.⁶

The deductible contribution limits under Section 404(a) also apply as if all group members were a single employer, and the total deduction is allocated among members under IRS regulations.³

If the Plan Fails Testing

When a plan fails nondiscrimination testing because the controlled group wasn’t accounted for, the correction window is short. Excess contributions must be distributed within two and a half months after the plan year ends. Miss that date and the employer owes a 10% excise tax on the excess. Fail to correct within 12 months of the plan year’s close and the plan’s tax-qualified status itself is at risk, which would expose every participant to immediate tax on their account balance.⁷

Health Coverage: ACA and COBRA

Brother-sister status also decides whether you’re an Applicable Large Employer under the Affordable Care Act. The ACA’s employer mandate starts at 50 full-time employees including full-time equivalents, and controlled group members’ workforces are combined for that count. The IRS states this plainly: companies related under Section 414 are a single employer for ALE purposes, and once the combined count hits 50, every entity in the group falls under the employer shared responsibility rules, including a member that only employs a handful of people.⁸

The same aggregation works for COBRA. Federal COBRA applies to employers with 20 or more employees, measured at the controlled group level. Two companies with 12 employees each wouldn’t trigger COBRA on their own, but as a brother-sister group with 24 combined employees, both have to comply.

Shared Tax Limits

Several tax benefits that would ordinarily belong to each company separately have to be shared among controlled group members. The IRS’s purpose is to stop owners from multiplying tax breaks by splitting a business across entities.

Section 179

The Section 179 deduction lets businesses immediately expense qualifying equipment and property instead of depreciating it. For tax years beginning in 2026, the deduction is capped at $2,560,000, with a phase-out starting once total purchases exceed $4,090,000. For a controlled group, all component members are treated as a single taxpayer, and the dollar cap must be apportioned among them.⁹ If Company A uses $2 million, Company B gets no more than $560,000.

Accumulated Earnings Credit

Corporations can generally retain up to $250,000 in accumulated earnings, or $150,000 for certain personal service corporations, before the accumulated earnings tax kicks in. In a brother-sister controlled group, that credit has to be split among members. The group may allocate it under an apportionment plan or divide it equally if no plan exists, but the total across members can’t exceed what a single corporation would get.¹⁰

Qualified Business Income

The Section 199A qualified business income deduction, available to owners of pass-through businesses, allows an optional aggregation election for commonly controlled trades or businesses. Aggregation combines wages and capital across related businesses when applying the deduction’s wage-and-capital limitation, which can sometimes produce a larger deduction than running each business separately. To aggregate, the businesses generally must share common ownership and meet at least two of three relatedness factors: similar products or services, shared significant business resources such as staff or facilities, or operation in coordination.

Schedule O Filing

Every corporation that’s a member of a controlled group files Schedule O (Form 1120) with its income tax return. The form reports how the group is apportioning its shared tax benefits, including the Section 179 deduction and the accumulated earnings credit. The filing is required every year the corporation is a component member, whether or not the apportionment changed from the prior year.¹⁰

If the group has an apportionment plan, meaning a written agreement among members on how each benefit is divided, that plan controls. Members don’t have to split evenly, and different benefits can be allocated in different proportions. Without a written plan, the IRS requires equal division. Each member keeps a signed copy of the agreement in its records; the agreement itself should not be attached to any member’s tax return.¹⁰

1
Office of the Law Revision Counsel. 26 USC 1563 – Definitions and Special Rules
2
eCFR. 26 CFR 1.1563-1 – Definition of Controlled Group of Corporations and Component Members and Related Concepts
3
Office of the Law Revision Counsel. 26 USC 414 – Definitions and Special Rules
4
Internal Revenue Service. Chapter 7 Controlled and Affiliated Service Groups
5
Internal Revenue Service. COLA Increases for Dollar Limitations on Benefits and Contributions
6
Internal Revenue Service. 401(k) Limit Increases to $24,500 for 2026, IRA Limit Increases to $7,500
7
Internal Revenue Service. 401(k) Plan Fix-It Guide – The Plan Failed the 401(k) ADP and ACP Nondiscrimination Tests
8
Internal Revenue Service. Determining if an Employer Is an Applicable Large Employer
9
Office of the Law Revision Counsel. 26 USC 179 – Election to Expense Certain Depreciable Business Assets
10
Internal Revenue Service. Instructions for Schedule O (Form 1120) Consent Plan and Apportionment Schedule for a Controlled Group