Sister [33F] cheated on bf [32M]

Throwaway account for reasons.
So, a couple days ago, I found out my sister was cheating on her boyfriend of several years for at least the last couple months. He had caught her kissing some other guy sometime in August, she promised it would never happen again, but apparently, just continued talking to the guy. From what I saw (in texts screenshotted from her boyfriend) it was at the very least emotional cheating and sexting, but it wouldn?t surprise me if it went further.
Her boyfriend is a man who I?ve found kinship with over their years as a couple, particularly the past year or two as he was extremely supportive during a particularly stressful time for my whole family. I probably talked to him more on a daily basis than most of my own family. I don?t doubt he?d made mistakes in their relationship, but from all I?d experienced from him firsthand, he?s a great guy that was constantly trying to improve for my sister. Definitely the only of my sister?s boyfriends that I would have been happy to have as a brother-in-law.
As for my sister, we have always been very close. I have always rooted for her happiness. But not at the cost of someone else?s pain. Her actions were so cruel and selfish. I am appalled by them and ashamed to have to even be adjacent to it. I never thought she would lack the integrity to not just break up if she was unhappy with her boyfriend.
So now, I?m not only sort of grieving the loss of someone I considered a close friend, but I?m also deeply shaken and feel betrayed by my sister?s actions. It?s like I don?t even know who she is anymore. I thought she was better than this. She?s my sister, I know she?s not perfect, but to see her seem so fine with seriously hurting someone she proclaimed to love for simple pleasure truly disgusts and angers me. I feel like I can?t even look at her.
The thing is, I live with her. I can?t exactly go away to get some space from her. And regardless of me not approving of what she?s done and wanting to hold her accountable, I don?t want to make life together hard and I don't want to throw away my relationship with my sister. How do I both hold her accountable and start to forgive her?
TL;DR: Sister cheated on bf that was like family. I am angry but want to forgive her and don't know how.
EDIT: To elaborate, her boyfriend found the texts the other night, he broke things off with her after finding the texts, then sent me screenshots of the incriminating texts. Partly as a way to say, "This is why you won't see me around anymore." and I think partly because he thought my sister would lie about her culpability in their breakup.


Source.