Girlfriend has feelings for my brother....?

I have a big problem. My girlfriend has feelings for my brother. I've always been the "provider" type guy... the one women claim to seek out for marriage material... AFTER they've had their fun with the "bad boys". Boo hoo for my girlfriend, she never really dated in high school so I'm her first boyfriend really. My brother is a little younger than me (I'm 21, he's 18, she's 20) and still looks like a punk/emo kid. He's always flirting and dating around plus he seems to have way more in common with my g/f than I do. Problem is I've been with this girl over a year. I practically live with her and I take care of her. She and I both know if she ever went out with my brother, it would end in heartache because he's a womanizer. But for some reason, she can't stop dreaming about him and wondering what it'd be like to be with him. Anyone else been in this situation? Oh btw, it's not just a little phase. She's felt this way about him from the moment she saw him.