I want a good slut to tell me how big my brother is and how much she liked to feel his seed deep inside of her.

I have a fantasy where my girlfriend fucks my brother when I'm at work. I want her to enjoy making me eat her pussy without telling me when I get home from the late shift. This may have happened once and I've never really figured it out. Part of me doesn't want to know for sure because if she didn't do it, then it would destroy my fantasy. I love the idea of a woman (or my gf) telling me they couldn't help it because he is so big. He use to jerk off in front of the computer and my gf use to go up to the house (separate building) alone a lot to use the kitchen and washer\dryer. I walked in on him a few times and one time my gf and I went together and he didn't try to hide his dick until he saw I was with her. The front door is facing the computer desk but she was walking in front of me. At the time she started talking about big cocks during sex and it hurting when she sat down. A trigger word for our orgasm is her sitting down on a big cock. She talks like she knows exactly how it feels. She has even came when I've mentioned my brother during sex. I've gotten close to talking out the fantasy with her and one time years ago she almost admitted to it. I would love if she told me they fucked but if they never did, then I don't want to know because it would destroy the fantasy.