Do I cut then out of my life?

Earlier this year a guy friend and I had a think going on (no sex) for like 15 days. It was all kept a secret from my twin, but she found out and got mad. The guy wanted to fix things with her so he distanced himself from me even after he said that we were going to try and figure out what was going on between us. He finally agreed to talk, I asked if he still like me and he said that he was trying to figure it out.

Two weeks later I asked what was going on and he finally admitted that he liked my sister all along and that what happened between us was only because he got caught up in the moment. I saw my sister and him kiss 3 days later and I got mad. I talked to him and her and they want to have a relationship and said that they would want to start it right away. After talking some more with them and making them realize that I’m really hurt they gave me a month to get it in my head.

What do you guys think? Is this something my sister should be doing to me or am I overthinking?

I also told him that I considered not being friends anymore but that I can’t do that since he already knows so much about me and it feels like we have known each other for a long time (we have only known each other for a little over 3 months). He also tells me he cares a lot and he also shows it, he’s there whenever I need him.

My sister also told me that if I’m not friends with him anymore that she wouldn’t care and that she would still date him...