Sister Aunt

When your mother starts having babies before your granny stops having babies, and now you are 3 years older than your aunt, but you're both raised like siblings.

Mama if you get pregnant now, my child will be a sister Aunt.

older sister aunt

A person which is not blood related to your family, but acts like both an older sister and a cool aunt. She fits right in to your family as if you are blood related.

Lara is our older sister aunt.