double aunt

noun2

1. •
   A woman who is one's aunt in two ways, e.g. being a sister of one parent and the wife of a sibling of the other.

   • The youngest son of Frikkie's sister had been betrothed to the girl who had measles in my house ; she was my sister's child, and thus I was double aunt. — 1923
2. •
   (rare) A woman who is the aunt of exactly two people, especially when those two people are twins.

   • Proving that when good things come they come in pairs, Moss Kelly (9) was the twice proud father or twin boys last Thursday . . . this makes Louise Splane (3) a double aunt or something. — 1944