The sister was a dark, shy and quiet girl. They were in a forest or a camping site while they were swimming, and the sister walked off and was grabbed by some dude in a van and almost raped. The Main came in just in time to sock the guy and save his sister. As they were walking away he put a towel over her shoulder. There was some lust between them. The story was found on sexstories.com, which is the new erotic literature domain of xnxx.com. Because I saw it a while ago, I might have actually found it on xnxx.com itself.
Source: reddit.com/r/eroticliterature/comments/4ox263/request_story_i_found_a_while_ago_main_character