What do you think about me being friends with someone my sister hates?
For real? Was that why you had a fight with your sister?
It is, and i think she's wrong.
You should be able to be friends with who you want to be friends with, but it will cause problems in your family. You are expected to be loyal to your sister because she will be your sister for the rest of your life but friendship can end.
I don't care.
That's fair and I can't stop you. Just try to keep in mind that you may be betraying your sister by doing this and you should be loyal to your family members. Really consider if it's worth it.