<human>: Provide a brief overview of the text.

I debated even posting this because I'm not sure if it is even my business, but /r/relationships had helped me a lot in the past with my own problems! 

My brother was dating a girl for about a year until late 2013. He was 17 and she was 15. She broke up with him when she decided that she should see people more around her age- she is very popular and has a "following" of guys in the high school. Unfortunately for my brother, this means that she likes male attention and wants to keep him around. They still text each other.

Since then, my brother dated one other girl for around a month before being dumped again and going back to moping over his ex. Two days ago, he heard that his ex-girlfriend had sex with a boy from the high school, and he has made actual plans to fight him in the upcoming week. This struck me as really really creepy and possessive and potentially could land him in jail.

My brother has no problem finding girls who like him, as he is a 6'2 hockey player so it seems weird that he is still caught up on this girl. 

Since he is an adult now I'm just worried that his emotionally-charged decisions now could follow him for a long time. Now that I'm home from college I want to be a good influence on him, but I don't know where to start.
<bot>: Brother is acting sort of creepy/stalkerish toward his 15 year old ex-girlfriend of 5 months. I don't know if I should intervene.