AITAO for not wanting to attend my sisters graduation after she entertained my husband romantically?

My bf and I have been together for two years, we have a 9 month old daughter. My immediate younger sister, J is my favorite out of 4 and we are very close. When I get pregnant I was staying with J and my bf would come visit often. We moved in together with my bf once I gave birth.

My bf and J have always had a banter kind of relationship. I have never doubted anything but I once saw how they text. My bf was flirty and my sister played along but didn't flirt back. That was way back and I never said anything.

About a month ago, a thought crossed my head that my bf could have a crush on my sister. I joked about it and he denyed but I couldn't shake the feeling. A few weeks ago I invited my sister over. She came. I was playing games on her phone when I got the urge to check their messages. They were texting and my bf made a clear move on her some time back. When my bf made a move , we were on a break, but it doesn't matter. I confronted the both of them. J was silent and my bf admitted to having a crush on my siz. It broke me💔. How my bf could fall for my sister and my sister for not stopping him. She never said yes but never stopped talking to him. She's 19 btw. She never told me. I understand if she couldn't tell me but what I can't is why she never cut him off. I believe she fed from his attention, and anytime he'd go silent, she would come visit me and remind him of her existence.

My sister left the next day. I have never talked to her, it's been almost 4 weeks now. I forgave my bf, we had a conversation and he's a good man, I know he regrets. We have never talked with J. She's graduating in 2 days and I don't even want to see her face. I can't even look at her photos. My parents are telling me to grow up and let it go and show up for the graduation .But it hurts. First, my man then my favorite sister. I know many of you will be against my bf but if you have ever forgiven your partner for hurting you so bad, please don't come at me. It still hurts. I don't know how to heal 😭 I don't know how to move on. I don't know how to look at my family after all the embarrassment. And I don't know how to forgive my sister, and right now, I don't want to. Someone please help