I'm a straight female in my mid-20s. I've been dating a wonderful guy for two years—but I recently found something that has put me on edge. Before we met, he was in a relationship with a terrible, alcoholic, and mentally unstable woman. They got pregnant early in the relationship and stayed together for about five years. We met a year after they broke up. I felt like I'd come to terms with the ugliness of his past, with his trying to stay in a bad relationship for the sake of his child and the rest of it. But recently, thanks to the vastness of the internet, I came across a suggestive photo of my boyfriend with his ex's sister. I asked him about it, and he admitted to sleeping with her while he was with his ex. He says it was during a particularly bad period, he was very drunk, she made the first move, etc., but I'm just so grossed out. Cheating is one thing, but fucking your girlfriend's sister? And it's not like this was a 19-year-old's mistake; he was near 30 and the father of a child. He also fudged a little about whether it was just one time or a few times. I feel like now I'm questioning his integrity. This is something that I wouldn't have thought him capable of doing. What do I do?