I don't want to end my friendship with my best friend after he cheated on my sister
Why would you still support him after he treated your sister poorly? I'm stunned.
It might seem a little stupid to some people, but I think that this has happened a lot of times before and it's going to happen again.
I understand you like your sister's ex, but he not only cheated on your sister but has not guilt about it. Is she OK with you maintaining a friendship with him?
As long as she is fine with it, I don't mind being his friend.
Maybe this is something that can heal over time but you should chose your family over your friends. She might feel rejected if she see's you hanging out with him.