Mix Up
I want to fuck him again, and I want to tell my sister what had happened. We shall see.
- That's Juicy! 89
- Remove It! 3
If you're unattached, go for it - the sex with the brother-in-law's father that is. There is no point in directly confronting your sister about riding her father-in-law's big cock.
Find some way to chat with him discretely for a few minutes, tell him about his mistake and tell him you want more of what he has between his legs. Reassure him that you have no intention of making life difficult for him or your sister, you just want regularly scheduled fucking with that huge cock.
Obviously, if your sister approaches him about why he's not fucking her as often he'll likely spill it all and you'll get an angry, upset, scared phone call or visit from your sister. Stand firm. Tell her you have no plans to take sole custody of her FIL's cock, but you'd like to get pounded with it a couple or more times per week. Don't overtly threaten since you don't need to. It's not like she can run off to her husband or your mom to complain, especially since you're willing to share. Just get the idea across clearly that you want to share, and the rest should take care of itself.