In legal usage, consanguinity is a relationship by blood — two people who share a common ancestor — and affinity is a relationship created by marriage, which makes you legally kin to your spouse’s blood relatives. Both concepts matter because American law assigns a numerical “degree” to every family tie, and that number controls who inherits, who can marry whom, who can sponsor a relative for a green card, who can make your medical decisions, and a long list of other outcomes.
What Consanguinity Means
Consanguinity covers biological kinship. The law splits it into two branches.
Lineal consanguinity runs straight up and down the family tree: parent, child, grandchild, great-grandchild. Each link is one generation removed from the next. Your parent is a first-degree lineal relative, your grandparent second-degree, your great-grandparent third-degree.
Collateral consanguinity covers people who share a common ancestor but do not descend from each other. Siblings share parents but neither descends from the other. Aunts, uncles, nieces, nephews, and cousins all sit on collateral branches.
Blood relationships are permanent. No court order, divorce, or other proceeding dissolves them.
What Affinity Means
Affinity is the legal bond marriage creates between you and your spouse’s blood relatives. The law treats you as standing in the same degree of relationship to those relatives that your spouse does by blood. Your mother-in-law is a first-degree relative by affinity because she is a first-degree blood relative of your spouse.
One boundary catches people off guard: the law generally does not recognize “affinity of affinity.” Your blood relatives are not legally related to your spouse’s blood relatives. Your brother and your spouse’s sister share no legal relationship under standard affinity rules, however close they may be in practice.
What happens to affinity when a marriage ends is not uniform. Most jurisdictions treat divorce as terminating affinity, though some maintain it when the marriage produced children. The rule at a spouse’s death varies the same way: the majority position is that affinity ends unless there are surviving children of the marriage, while a minority of jurisdictions keep it intact regardless. Federal tax law is cleaner. The IRS treats relationships established by marriage as surviving both death and divorce, so a former in-law can still count as a qualifying relative on your return.1Internal Revenue Service. Publication 501, Dependents, Standard Deduction, and Filing Information
How Degrees of Relationship Are Counted
Federal law uses the civil law counting method, and it is the dominant approach across American legal contexts.2Office of the Law Revision Counsel. 28 USC 455 – Disqualification of Justice, Judge, or Magistrate Judge You count one degree for each generational step between the two people.
For a direct line, the count follows the chain:
- Parent to child: 1 degree
- Grandparent to grandchild: 2 degrees
- Great-grandparent to great-grandchild: 3 degrees
For collateral relatives, count up from one person to the nearest shared ancestor, then back down to the other person:
- Siblings: 2 degrees (up to the shared parent, down to the sibling)
- Aunt or uncle to niece or nephew: 3 degrees
- First cousins: 4 degrees
For affinity, treat your spouse as occupying your position on the chart. A brother-in-law is a second-degree relative by affinity. A spouse’s first cousin is fourth-degree.
A Note on the Canon Law Method
Some religious institutions and older legal texts use the canon law method, which counts only the longer side of a collateral relationship instead of adding both sides. That makes first cousins second-degree under canon law but fourth-degree under civil law, and siblings first-degree rather than second. Both systems agree on lineal counts. Unless you are dealing with ecclesiastical marriage rules, the civil law numbers are the ones that govern.
How Adoption Changes These Relationships
Adoption creates a complete legal substitute for the biological parent-child relationship. An adopted child is treated in every respect as if born to the adoptive parents and gains full consanguinity with the whole adoptive family for inheritance, marriage prohibitions, and every other context where degrees matter. Federal tax law gives full effect to legal adoption when determining family relationships.3eCFR. 26 CFR 1.267(c)-1 – Constructive Ownership of Stock
The other side of that rule is that adoption generally severs the legal relationship between the child and the biological parents, along with the inheritance and other rights that flowed through them. Stepparent adoption is the main exception: only the non-custodial biological parent’s side is cut off, and the custodial parent’s family stays intact.
Where Degrees of Relationship Actually Matter
Inheritance Without a Will
When someone dies intestate, courts distribute the estate by degree of consanguinity. The Uniform Probate Code, adopted in some form by roughly a third of states, sets a cascading priority: surviving spouse, then descendants, then parents, then siblings and their descendants, then grandparents and their descendants (aunts, uncles, and first cousins). If no relatives can be located within the statute’s reach, the estate escheats to the state. Whether a first cousin qualifies as an heir or is skipped turns on the specific statute and on which closer relatives survived. States that have not adopted the UPC use their own lists, but the closest-first structure is nearly universal.
Who You Can Marry
Every state prohibits marriage between people within certain degrees of consanguinity. Parent-child and sibling marriages are banned everywhere. Most states extend the prohibition to aunts, uncles, nieces, and nephews. First-cousin marriage is where states diverge: roughly half allow it, some allow it only under specific conditions such as age minimums, and others prohibit it outright. Violations are treated as criminal offenses in most states, with widely varying penalties. Some states extend prohibitions to certain affinity relationships too, such as stepparent and stepchild, on rationales tied to household integrity rather than genetics.
Family-Based Immigration
Immigration law splits family sponsorship into two tiers. Immediate relatives of U.S. citizens — spouses, unmarried children under 21, and parents of citizens who are at least 21 — face no annual visa cap.4Office of the Law Revision Counsel. 8 USC 1151 – Worldwide Level of Immigration Everyone else falls into the capped preference system:
- F1: unmarried sons and daughters (21 and older) of U.S. citizens
- F2: spouses and children of lawful permanent residents, and their unmarried adult sons and daughters
- F3: married sons and daughters of U.S. citizens
- F4: siblings of U.S. citizens, if the citizen is at least 215Office of the Law Revision Counsel. 8 USC 1153 – Allocation of Immigrant Visas
The hard boundary sits at siblings. There is no family immigration category for grandparents, aunts, uncles, or cousins. A citizen wanting to sponsor an aunt or a first cousin has no direct pathway to do so.6USCIS. Green Card for Family Preference Immigrants
Taxes and Related-Party Rules
The IRS definition of a “qualifying relative” for dependent purposes reaches further than most people expect. You can potentially claim a parent, sibling, niece, nephew, aunt, uncle, or in-law without that person living with you, as long as the income and support tests are met. Someone outside those listed relationships can still qualify, but only if they live with you the full year.1Internal Revenue Service. Publication 501, Dependents, Standard Deduction, and Filing Information
For business owners, family attribution rules can treat you as owning stock actually held by your spouse, siblings, ancestors, or lineal descendants, which pulls transactions into “related party” territory and restricts loss deductions. The rules do not extend to aunts, uncles, in-laws, or cousins.3eCFR. 26 CFR 1.267(c)-1 – Constructive Ownership of Stock
Public Employment and the Courtroom
Federal law prohibits a public official from hiring, promoting, or advancing a relative to a position in the agency the official controls. “Relative” is defined broadly, covering blood, marriage, half-blood, and step-relationships out to first cousins and in-laws. An appointee hired in violation is not entitled to pay.7Office of the Law Revision Counsel. 5 USC 3110 – Employment of Relatives; Restrictions Most state and local governments have parallel statutes, often reaching the third degree.
Federal judges must recuse from any case where a party, attorney, or likely material witness is related to the judge or the judge’s spouse within the third degree, counted under the civil law method. That reaches parents, children, siblings, grandparents, grandchildren, aunts, uncles, nieces, and nephews. A failure to disclose can produce a mistrial or reversal.2Office of the Law Revision Counsel. 28 USC 455 – Disqualification of Justice, Judge, or Magistrate Judge
Medical Decisions and Family Leave
When a patient cannot make medical decisions and left no advance directive or healthcare power of attorney, most states default to a surrogate priority list: spouse, then adult children, then parents, then adult siblings, sometimes extending to grandchildren, grandparents, and close friends. Degree of relationship decides who makes potentially life-or-death calls, which is a strong argument for putting an advance directive in place instead of leaving the decision to statutory defaults.
The Family and Medical Leave Act limits protected leave to caring for a spouse, child, or parent with a serious health condition. Parents-in-law, siblings, and grandparents are not covered. “Parent” includes anyone who stood in a parental role to you as a child, and “child” includes stepchildren, foster children, and legal wards, but the child must generally be under 18 or have a disability. Military caregiver leave uses a broader “next of kin” standard.8U.S. Department of Labor. Family and Medical Leave Act
Social Security Survivor Benefits
Social Security restricts survivor and dependent benefits to a narrow group. A spouse can collect on your record if the marriage lasted at least nine months before death, and a divorced spouse qualifies if the marriage lasted at least ten years.9Social Security Administration. Survivor Benefits Unmarried children under 18 (or under 19 if still in high school) are eligible, along with adult children with disabilities that began before age 22. A dependent parent aged 62 or older who relied on the worker for at least half of their support can also collect.
The list stops there. Siblings, aunts, uncles, and cousins have no eligibility regardless of financial dependency, and grandchildren qualify only in limited circumstances. The pattern holds across almost every area of law that touches family: the closer the degree, the more rights flow from it, and the drop between qualifying and not qualifying is often abrupt.