I'm thinking of choosing my friend's wedding over my sister's
I am not sure I understand. Why would you choose your friends wedding over your sister's?
I hate my sister and she doesn't like me as much as she likes you.
Even though it is a difficult choice, you should prioritize your sister's wedding over your friends. You should be there for family even if you don't have a great relationship.
I don't want to. I don't care about being there for family.
Don't you want your family to be there for one another? If you do not attend your sisters wedding it will upset everyone for some time.