Not without the consent of her parents or court order.
no
Uncle is to aunt as father is to mother.
Sister-in-law describes a female. Brother-in-law describes a male.
The opposite relationship of uncle would be Niece or Nephew.(The opposite gender is aunt.)
The son of your aunt and uncle is your first cousin.
If your aunt has a brother who is not your uncle, then your aunt is the wife of one of your parent's brothers, and that aunt's brother is no relation to you.
Great great aunt/uncle. To your niece, you are the aunt/uncle, to her child you would be the great aunt/uncle. So you would then be the great great aunt/uncle of her child's child (her grandson)
Your niece will call you her aunt or uncle.
I saw your aunt and uncle
In spanish, aunt is tia and uncle is tio.
Your uncle's sister is your aunt, or possibly your mother.
In Portuguese aunt is spelled Tia (you pronounce it chia) and uncle is tio(you pronounce it chi-o)