This movie centers around a group of teenagers, one of whom has a mentally disabled older (mid-20’s) brother/relative. One of the girls begins a relationship with the disable man (after seeing his giant member through his long underwear during an underwater swimming contest in a canal/river). I think it was some kind of pity case to deflower him, but she couldn’t take the beer. I think the girl is then killed by another man who finds out about the relationship (and maybe demands sex for his silence?), but the disable man is blamed.
I don’t think it quite matches up, but your description reminded me of Waterland.
http://www.imdb.com/title/tt0105790
Reminds me of Straw Dogs.