My boyfriend slept with my sister, what should I do?

Ok, so let me fill you in on the situation. I was with my boyfriend for 6 years straight, it was never an on again off again type relationship. We have a 2 year old daughter, we had what I THOUGHT was the perfect relationship, never had any major problems, we lived together for 5 years, he’s always been there for me and I’ve been there for him, he was part of my family, everyone loved him, he had no other family of his own, except his mom and sister who he doesn’t see often anyways.

He was my best friend. As for my older sister, we always got along, I was closer to her than any of my other siblings, never fought or anything, didn’t hate each other, when she needed money I gave it to her, a ride, I was there. I helped her with EVERYTHING.

Well, in November I found out that my sister and my boyfriend were sleeping together, not once, not twice, but through a period of a whole month, she also bought another cell phone so the two could keep in contact. I found out because her boyfriend was going through her purse and found the phone, I’ve read all the message which were clear that they were having se.x.

A week when by and I got none stop messages and phone calls from the boyfriend saying he’s sorry and blah blah blah… we talked it out and he told me EVERYTHING… even stuff I didn’t want to hear. But anyways its been a little over 2 months since everything came out in the open, me and the EX-boyfriend are no longer together but he is still in my life because of our daughter, he’s a great dad by the way just a sh!tty boyfriend… like I said it’s been over 2 months and I still have not gotten a single word out of my sister, who knows my number and where I live, she hasn’t shown any remorse for what she’s done since the day it came out, the only thing I’ve heard from her is cussing and accusing me of sleeping with her boyfriend to get ‘revenge’… which I would never do because I have more respect for myself.

Anyways, I really wish I could forgive my boyfriend because I miss my family and it just doesn’t feel the same … a part of me says to forgive and forget and another part is saying he will cheat again…

I told him if it was some random girl MAYBE I could forgive him but this was my sister! WHO DOES THAT!!