I recently reunited with a former high-school girlfriend and we’d been having a great time until I told my asshole brother about her a few weeks ago. He flippantly responded that he’d had sex with her shortly after she and I broke up. I’ve asked her about it and she says that although they dated a few times, they never slept together. I honestly don’t know who’s telling the truth, and it’s bothering me a lot. Now it’s hard to take her seriously, like I can’t imagine bringing her to a family function anymore.

He Said: I’m not sure if this is helps, but my research team informs me this situation has been covered ad nauseam on the Young and the Restless. For actual humans, there’s usually a deep-seated evolutionary revulsion against having sex with someone whom a close family member has also mated with. Evidently your brother lacks this gene, whether he’s bullshitting about boinking her or not. My advice is to give yourself some time, try to enjoy each other, and stay open to the possibilities of a long-term relationship. Every couple encounters seemingly intolerable issues, but successful couples deal with them as they come up and move on together. You may, however, want to ask your girlfriend if she dated anyone else you know in the intervening years, like your dad. That could make for a very interesting holiday dinner.

She Said: There are three parts of you that need to be polled on this. It sounds like your head is saying “I can’t take seriously a girl who dated/slept with my brother.” Your gut, meanwhile, is understandably grossed out by the idea. But what does your heart say? If I were you, my biggest reaction would be the confusion and pain caused by the fact that one of two people I care about is definitely lying to me. I don’t know if you should keep seeing a woman who slept with your brother more than a decade ago, but I do know that you probably won’t rest until you at least get the truth.

One approach is to earnestly tell your girlfriend that if she’s fudging the truth, you need to know now so that you don’t find out much later. Another approach is to get them both in the room together. Once you know the truth (or use your gut to intuit it as best you can, based on their responses to you challenging their contradicting stories), you’ll know what to do next. If she slept with him, I’d advise trying to let it go, just like you do a lot of your high-school behaviors and memories. It's not like she cheated on you with him. But only you know if you have a big enough heart (and brain and stomach) to do that. This is something most people would struggle with, so don't feel bad if you can't handle it. If she didn’t sleep with him, the way is cleared for an easier relationship with her—but I'd advise avoiding your brother altogether.