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.
you should call your daughter Lexi, but I think that Rex is more of a dog's name.
Your niece.
Niece
Aunt
Your sister's daughter is your niece and her granddaughter is your great niece.
Your grand daughter.
You would call your sister's son your nefu and her daughter neice.
Great Grand daughter.
Your sister's daughter is your niece, her granddaughter is your "great niece"... or you could just call her by name.
Your sister's daughter is your niece, her granddaughter is your "great niece"... or you could just call her by name.
Jackson Jakob Michael James Benjamin
Your sister's daughter is your niece. You are her aunt if you are female, or her uncle if you are male.