Sister’s husband cheated with an ex and got her pregnant

My sister’s husband cheated on her with an ex girlfriend and got the ex pregnant. My sister decided to work on the relationship and they’re not getting a divorce. They (sister&husband, NOT husband&ex) have children together and the ex had an abortion so those are also factors in her decision, but I just don’t think I could live with that decision, knowing how deeply my husband betrayed me and yet I decided to stay. Just think about it: you have one precious life and you decide to put up with this? I think my sister would be better off alone or with someone else. What would you do in a similar situation?

ETA: I’m not criticizing my sister, I support her in whatever she thinks is the best for her.