co-aunt

Your brother's or sister's sister-in-law, or your niece's or nephew's other aunts that aren't your sisters.

Liz and I aren't blood related, as her sister married my brother, but we are co-aunts to the same children.

co-uncle

Your brother's or sister's brother-in-law, or your niece's or nephew's other uncle(s) that aren't your brothers.

Jonathon and I aren't blood related, as his sister married my brother, but we are co-uncles to Jack and Caroline.

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