My sister used to fuck our dad. I actually caught them once when I was 16 (sister was 18). But I'd suspected it for a couple of years at that point... pretty sure it started around the time our parents got divorced.

Seemed to be 100% consensual as far as I could tell... my sister certainly seemed to be enjoying herself that time that I caught them, and knowing her I'd bet money she was the one who instigated the whole thing in the first place.

I'm not sure if they're still doing it. My sister moved out when she was 22 and has lived with a string of boyfriends since then. But every time she goes through a breakup, she always goes back and ends up living with Dad for at least a few months.