You are not related to your boyfriend's sister, although you and she may be friends. If you were to marry your boyfriend, she would be your sister-in-law.
It is not a crime to have 2 boyfriends.
Real sister, half sister, or step sister
sister in law.
My Boyfriend and my cuusin
Your sister's sister is your sister.
No relation whatsoever. You will be an aunt or uncle to your sister's child and your brother-in-law's sister will be the child's aunt, but that's as close as it gets.
Your mothers sister is your Aunt
There is no relation. Your not blood at all so your just really good friends.
sister-in-law
She would be the same relation to you as she is to him.
No relation...lol
Yes