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.