What should you call your sister-in-law's daughter?

If your uncle is the brother of one of your parents, his sister-in-law is either the sister of his wife or the wife of his brother.

If his sister-in-law is the sister of his wife, she is not related to you, and neither is her daughter.

If his sister-in-law is the wife of his brother, then she could be your mother and her daughter would be your sister, or you if you are a girl. Otherwise she is your aunt (the wife of another brother of one of your parents) and her daughter is also the daughter of a brother of one of your parents and thus your first cousin. If the daughter is from a previous marriage, then she is not related to you, but you may choose to treat her as a first cousin, especially if she was very young when her mother married your uncle.