Okay, I understand that they didn't know they were related in the beginning, that's understandable...
...However, once they found out the truth, they continued doing it...
...Let me take my ass back to sleep. It's too early for this ****...
They can do whatever they want. But had that been me in that situation, I don't care how good the oral/vaginal/anal/handy-dandy wrist game is, I'd be done. Like...bye.