Why not? I personally would never do a sister. That’s a bit much but I can tell you I personally know multiple related brother/sister couples that secretly messed around. I even know one because I found a video online of them literally having sex. And the video was uploaded by my friends email and said “brother convinces sister to finally suck his cock”. In the video she gives her own brother a handjob and then sucks him after he keeps pushing her head down onto his cock then she eventually stops resisting and starts sucking and then towards the end of the video she decides to get on top and ride him without a condom for like 2-3 minutes. The video was taken down after I told my close friends to let the brother/sister know i saw it. The sister didn’t know her brother posted it online. Me, personally, I won’t lie- I found out my cousin had huge tits (never knew) after went shopping and she bought a 34GG bra. I freaked out and after some convincing I got her to show them to me then later to let me touch them then eventually so suck them and after a month of nagging her we had sex after she sucked my dick. She kept saying sucking my dick was making her wet and she wished she could have me inside her so I finally convinced her to let me put it in. We fucked almost every day for 2 weeks before deciding to stop.