I don't personally know of any such relationships, but I do have a friend who mentioned she and her hubs had sex with his brother. The brothers are actually twins. They don't seem to have any ill effects from that.
I understand the impulse to want to talk it out, in every detail. I just want to offer the caution that you cannot *know* until you actually are in it. Like learning to swim. You can read everything written about swimming, and you can watch videos of it, and you can talk to people who swim; but until you get in the pool, you won't *know* (and then, you don't start out with the gold medal, you likely start in the shallow end, maybe learning to float, progressing to dog-paddle, and then learning proper strokes).
Very brave of you to even be considering it