It sounds like her stepbrother took advantage of her, because 13 is very young to have sex. Why do you think she was crying? It's not because she lied, well maybe partly, she had to, but more to do with the fact that she probably feels extremely guilty in a situation she wishes never happened. Kind of how rape victims feel after they are raped. Just because he isn't a skeevy older man, doesn't mean he isn't capable of inflicting that.
I don't know the details, but I can almost certainly say that this was not a mutual attraction. She was molested. Probably manipulated into this.
To the posters who say, maybe he's cool? Who are you kidding? This is an incestuous situation. Maybe he kept coming to her bedroom at night, and wouldn't leave her alone, so she gave in. Maybe she has no security now at home. That's a mental mind ****.
If that bothers you, then you need to grow the **** up. Something happened to her that she can't control, and hates herself for. She needs counseling. The fact is you are 18, and you engaged in a relationship with this girl. If you love her as much as you say, you need to do the responsible thing and support her. These things in life don't always come to you when and if you want them. For her sake, do not throw the towel in. I can see that only screwing her up more.
Sometimes I worry about being a success in a mediocre world
-Lily Tomlin