Who is right here?

Okay, so a few weeks back, my best friend slept with my ex. This isn't the matter of the question though it does have something to do with it. Since this has happened my twin sister has slowly but surely stopped hanging out with me and started hanging out with them more. Sure, they live closer. Sure we were all friends before. But now she won't even make time for me. She has two kids, my nieces, who I love very much. I can't see them without seeing her. I just want to know if I'm right or wrong about being upset with her about this. I've tried planning times to come visit her. Almost at least a week in advance and the closer the day comes that we have BOTH selected for me to go see her she comes up with an excuse or she tells me she will be hanging out with my ex best friend and my ex boyfriend, knowing that I don't want to be around either of them. She tells me she wants us to all be friends and then tells me I'm being immature by not wanting to be a mutual friend with the person cont