Yeah, I have two lost stories, and I think the author wrote more, so...
First that I remember opens with sister, who is the main character(?) and her friend sitting around doing lewd things to themselves and each other(intimate painting with pink lipstick I remember) while talking about hunk brother(and friend flaunting the works when brother comes home). Brother has an oriental girlfriend , and I seem to remember some sort of fraternity shenanigans or similar?
Second, not same family, brother is the main character. I think he and sister get it on in the first chapter, but what I really remember is the second, where hot-mom and he hang around the pool, he oogles her skimpy bikini and she is very smirk and doesn't mind at all, commenting on how men being all the same, oogling boobs and butt, and then(I think) the classical help with sun cream, and definitely him deep in her, giving mom a bit of a shock due to his size.
I stupidly never bookmarked, since I knew the stories and...then I didn't.