Uncle is to aunt as father is to mother.
Uncle Vernon is mentioned first. The first chapter is told from his point of view.
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.
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)
In spanish, aunt is tia and uncle is tio.
I saw your aunt and uncle
Your niece will call you her aunt or uncle.
Your uncle's sister is your aunt, or possibly your mother.
Your uncle's wife is your aunt.
In Portuguese aunt is spelled Tia (you pronounce it chia) and uncle is tio(you pronounce it chi-o)