lol, not exactly sure where you're coming from, but I have an attractive sister, and trust me there's no point at which I'm suppressing any thoughts. They just don't exist.
Cousins are a different story all together. Aunts, I find, somewhere in between. Although I've never ever fantasied about aunts, I have about cousins. Although obviously I'd never ever do anything.
You've noticed? Or you've been divulging in it?
Can't speak for anyone else but this truly doesn't enter my mind.
Although I do compare my girl to my brothers gf's. And hate it. Because they are all taller (younger) and better looking than me (truly, I'm not BDD).
Also, even though my sister is very smart, I find it hard to ever have any discussion on the problems guys have in dating. I think she has a hard time truly understanding how hard it can be for guys. In fact we only discussed it once, and she brought up how ridiculous it was that guys "felt the need" to go to the gym. "It's confidence that girls look for, not muscles!!!". I realised then that this is a discussion I can't have with her, at least not with her current views.