She obviously has some feelings for him and she's trying to hide it, due to your family's beliefs and her school.
The guys sounds like a dirtbag, but that doesn't mean he forced himself on her and you shouldn't call him a rapist based only on her confused story.
You need to understand if she wanted to refuse but felt mentally obligated or if she's just feeling ashamed of what she did with that idiot.
Hope it all turns out ok in the end.
Of course I'm considering that, which is why I'm letting him tell his side of the story as well. It's a complex situation which is why I want to approach this carefully. I did let her know fully that whatever happens, I will not judge her and will help her out completely and so the best way for us to move forward is if she gives me the complete and true story. I'm taking the details with a grain of salt, but it's impossible for me as her brother not to give into her side of the story. I'm cognizant of that bias, however.