What is a brother-in-law?
A brother-in-law is your spouse's brother or your sibling's husband — both senses are standard, and the same person can be both to different people at one table. The relationship is affinal, created by marriage: no shared ancestor, no blood, and the title formally lasts as long as the marriage that created it.
Relationship
Brother-in-law
- You
- Your spouse
- Their brother
From the other side, you are their sibling-in-law.
This connection runs through a marriage, not a shared ancestor, so the ancestor chart can't draw it.
The calculator below works out blood relationships through a shared ancestor — use it to explore the rest of your tree.
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This relationship sits outside the shared-ancestor model, so the card above carries the answer and the calculator opens in its default state. Work out any blood relationship with it, or open the full relationship calculator.
FAQs
Is my sister's husband my brother-in-law?+
Yes — and so is your spouse's brother. English uses one word for both marriage routes.
Is my wife's sister's husband my brother-in-law?+
Strictly no — he is your co-brother-in-law, since two marriages sit in that chain. Plenty of families use 'brother-in-law' loosely for him anyway.
Does the calculator store my family details?+
No — it runs entirely in your browser. Nothing you enter is uploaded or saved on our servers.
Start with one relative and see what comes back.
Add what you remember and AncestorIQ searches for the rest. Free to build your tree for as long as you like.
No credit card needed. Your tree stays yours.
