Fortunately, you don't have to like him. :)
I have several sisters, and they let me swim on my own as far as relationships are concerned. Maybe if I was sinking all the time they would have opinions, but they seem to respect my capability to handle life, (and I sure wouldn't want any of *them* jumping in and messing around with it.) The problem with telling your sister that you think she is involved with a bad guy is that you are essentially telling her that your judgment is better than hers. Between sisters, thanks to childhood sibling rivalry, this is not a friendly thing to say. Leave it to her girlfriends to tell her if the guy is a loser. They might not -- if she's happy and he is finally on the straight & narrow, that might be the end of the story.
Good points, I support her but can't really like him. They are not married, and live separately. Both have children from previous marriages and I just think that he has lied to so many people, her, his other woman, his children, her children, etc, etc I can't believe it will be long before he does it again. He said he would never see her again, then promplty did later that week. Albeit that was a year ago and he has seemed to be faithful therafter besides seeing her ocassionally for dinner, coffee.
I'm with specialmom; don't judge and try not to ask your sister. It might be working for her for whatever reason, and she might not want you in her business. Incidentally, I don't think he was cheating on your parents. He was cheating on his wife, and further dividing his attention meant less attention for the kids. But it doesn't have anything to do with the parents, unless, of course, before he married your sister he swore to them that he would never cheat. In that case, what he did to your parents was broke his word.
Who's to say what works for someone else? I've seen many couples overcome infidelity. Some go on to have really great relationships afterwards because they learned how much the other meant to them. Other times a couple will always have a wall between them. So without any info other than what you've written, I think the best thing for you to do is to try to forget he cheated yourself. Being supportive of their relationship is the right thing to do (especially since they have kids). They are making a go of it and I hope it works out. I'm sure it is hard as you might always wonder about him------------ (and she may as well)--------- but try not to think about it. I wish them luck and you so as well as you support them. peace.