The sister or sister-in-law of one's mother.

What's the difference between maternal and paternal aunt?

As adjectives the difference between maternal and paternal

is that maternal is of or pertaining to a mother; having the characteristics of a mother, motherly while paternal is of or pertaining to one's father, his genes, his relatives, or his side of a family; as , "paternal grandfather" (one's father's father).

Who is known as maternal uncle?

The brother or brother-in-law of one's mother.