Best friends dating younger brothers.

Ok so,
I met this girl back in middle school. We became really good friends, and remained really good friends through out 9th and 10th grade. She moved away for junior year. Then for senior year I moved, and I ended up at the same school that she was...

attending. So we started hanging out and she started coming over my house.
Ok well I have a younger brother. Im 18 and he's 16. The girl is my age, they met and secretly started dating. Secretly because she was my best friend and he was my brother and I guess they thought it would bother me. Which it did because she's older than him and more experienced. Also, I just feel there was a boundary she crossed with doing that.I would have much rathered him having a girlfriend his age because they would both be young and they could learn from each other. Anyway, I didnt interfere with anything. I just told her to be careful with my little brother and not to hurt him. As time passed I felt uncomfortable around them. It just seemed strange for me to see a girl I've known since middle school dating my younger brother. Not only that, but she was his first real girlfriend.
After some time she started telling him that she much rather be his friend and blah blah. I know I shouldnt get all mad but it effects me because I know them both very well. He doesnt really show his feelings like that, but I could tell its really effecting him, and she's just going on trying to hang out with me like nothing happened. Saying that he was young blah blah. If that was the case why put my younger brother through all that?
I dont know, its just really bothering me and I suppose I also needed to vent.
Comment if you wish

you seemed to take it very well. If one of my friends dated my little sister I'd break his legs. And dating secretly, doesn't sound like you can trust her very much. I can see why its giving you probelems, she did cross a line that should never be crossed without asking. Im actually in love with my friends little sister. Im almost 21 and she's 18. I would never act on it though because thats a slap to the face to my friend. And the fact that he would break my legs, lol. Seriously.

I have the same problem but it hasn't progressed that far...My friend that I've known for a long time is going out with my younger brother. She is 18 and he is almost 16..I would have no problem with it but I've seen how she operates..I love her to death but I just know its going to end badly...and I feel like by doing this she is just jeopardizing our friendship and me and my brother's relationship. She has always been pretty bad about dating guys then breaking it off...I've watched it happen many times but now it's like a slap in the face because its my baby brother...I don't know what to do

I like my best friends brother and she is mad at me for liking him... I think she dont think im good enough for him im so scared to ask him out because I dont wanna loose her as a friend... see shes 18 and hes 16... but he is two years older than me and I really think he is something special but I dont know what to do right now...