This is just a specialized and potentially even more toxic case of dating a best friend’s ex. I actually know a man who married his girlfriend of seven year’s sister. You can imagine family Thanksgivings are more than a little strained. Again, if the details but do tell the relative before they find out via the family grapevine. If at all possible, avoid this plight. If you can’t avoid it, make sure that your motives are pure, and you’re not acting out of some ancient feud of competition.