What Relation Would Your Father's Sister-in-law's Sister Be?

The sister-in-law of your father is the sister of the wife of your father. In other words, the sister-in-law of your father is the sister of your mother. Therefore the sister of the sister-in-law of your father would be your mother.

However, the sister of the sister-in-law of your father could also be your aunt (or any of your aunts) if the sister of the sister-in-law of your father has one or more sisters. The sister of the sister-in-law of your father could also be the sister of the wife of the brother of your father.

You and your siblings (that is your brothers and sisters) are the children of the sister of the sister-in-law of your father. However, if the sister of the sister-in-law of your father is your aunt, her child or children would be your cousin or cousins, and they would be the nephew and niece (or nephews and nieces) of your mother and her siblings.

Your father's sister-in-law's sister is your mother.

Aunt