That guy is not your best friend. He purposely excluded you, chose the possibility of sex over you, and went after an ex without even consulting or seeing how you felt about it. He's egotistical and thinks with only one thing, his dick, he's an asshole.

Yeah... that dude definitely doesn't deserve the label of "bestfriend". Probably doesn't even deserve to be called "friend".

I know it can be hard ending a friendship when your family and friends are all close. Trust me when I say do not put up with it. It may be difficult at first but it's way better for your well being. I stayed friends with someone who severely bullied me because our families were close and we were a tight group of friends, to this day I regret it because things never got better. You will be much happier without that person in your life.