My gf's mother had her when she was 15. They are both REALLY good looking and look like sisters. When we're all out together guys hit on her. It freaks me out. Do I defend her or what?

just be a spectator. I assure you, it's not the first time she's dealt with this. I think it's great. The benefit to you is (if you stay with her) this will be how your gf looks at that age. Awesome!!