DEAR WANTS WILL: Perhaps it's time to stop being the "good girl," begin acting like a woman who knows what she wants, and confronts double standard in your family. If your sister was "sophisticated" enough to have serial affairs, and your parents have been so worldly they have turned a blind eye to it, then they should all be adult enough to realize that you are entitled to your happiness, too.
Although this may make for some awkward first few family gatherings, as grown-ups, everyone should be able to get past it. But if they can't, you are going to have to decide whether you want this man, or to be a people pleaser for the rest of your life.