Your brother's wife is your sister-in-law, but she is your children's aunt.
aunt
She is your aunt. This is true in most traditions. English does not distinguish between an aunt by marriage (father's brother's wife) or an aunt by blood relation (father's sister).
In English, you call your maternal uncle's wife "aunt." It's not rocket science, honey. Just like how you call your paternal uncle's wife "aunt," keep it simple and call her the same thing.
aunt
That would be your great-aunt, by marriage.
Your mom's brother's wife would be your uncle's wife, or your aunt-in-law.
Your father's brother is your uncle. His wife is your aunt.
If your aunt is the sister of one of your parents, her husband's brother (her brother-in-law) is not related to you. If your aunt is the wife of a brother of one of your parents, then her husband's brother (her brother-in-law) is either your father or another one of your parents' brothers.
In English, the wife of your brother (elder or younger) is your sister-in-law. If you have a child, she is your child's aunt.
Aunt
A hoe.
She is your aunt. This is true in most traditions. English does not distinguish between an aunt by marriage (father's brother's wife) or an aunt by blood relation (father's sister).