My little brother's girlfriend is a little tramp?

Today my little brother brought over his girlfriend and I wasn't very fond of her attitude. From what I hear she has a history of sleeping with multiply guys. So anyway as I was going in the basement I saw her and my other little brother making out. She is begging me not to tell what I saw. However, I did tell him. So I slapped her and told her not to come back to my house. This girl is 17 years old and acting like the biggest . When I told my little brother about what I saw he laughed it off and said it was okay for her to act like this. This is his first girlfriend and I'm afraid that he is going to get hurt very soon by this girl. How can I explain to him that she isn't worth it and she is using him? Is it normal for a young boy who is in love to act like this?