I had a similar constellation in the backstory of one of my characters. It was male identical twins who were very close to each other. One of them was jealous of an uncle with whom the twin spent too much time. When the twins were studying, living together in a flat the jealous twin seduced the other one while they were both drunk. The seduced twin said they should be reasonable and not to this again, but the other one couldn't accept that and seduced him again and again. But when they had finished studying the jealous one became furious when the other one dropped him and married a woman. The jealous twin was heartbroken left the country, became a really violent brute who murdered and robbed people and was very feared.
So it was also a case of someone being possessive about his twinbrother.
On „normal“ boards (with sidewide plots, so no 1x1 roleplay) the staff demanded to edit the twincest out. That happened on boards with 3/3/3 ratings, where it was O.K. to describe the most brutal murders in every detail, but to have non-violent twincest in your character's backstory was a no-go.
I was pondering to roleplay a younger version of this character, looking for someone who plays the other twin. So that we roleplay the student years of the twins. But after getting so many appalled reactions for twincest in the backstory I rather wanted to spare myself further hassle and doubted that I would find anybody who wants to roleplay such a plot. I suppose that would be only possible on boards like Bluemoon.
I also had a character who was a vampire who had been born in the 18th century, and he kept track of the human family members. The vampire invited a direct descendant of his brother to the family residence. So the vampire was that young male human's Greatgrandgrandgrand...uncle. It was planned that the vampire falls in love with the young man, seduces him and turns him later on.
I started roleplaying this on a Googledoc, unfortunately the roleplay buddy ghosted on me. I would have liked to roleplay that with someone else but didn't find anybody.
The characters weren't that closely related. Even so most people didn't want to know me any more as soon as I suggested such a pairing – when I looked for 1x1 roleplay buddies on a recource site. That's one of the reasons why I'm back on a board where people are more open-minded for taboo-pairings.