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.