<human>: I am a 16M. I am on the brink of a relationship that may tear apart my relationship with my brother, if he doesn't decide to kill himself.

My brother(19) started dating a girl(18, lets call her C) about a 15 months ago and she broke up with him after about 3 months. For some strange reason my bro decided to become completely obsessed with her, and he has not left her alone for a year.

After my bro went off to college, C and I became close friends, and she told me about how awful he treats her. She tried to remain friends after they broke up, but he began verbally abusing her. He told her that she is stupid, he yelled at her about the guy she started dating (who had been one of my bro's friends), called her a slut and other terrible things, none of which was true. At the same time, he somehow still has an absurd idea that he will get back together with her, and he has threatened to kill himself if she didn't agree.

While mine and C's friendship started off as just me helping her out with my bro and her helping me out with my at-the-time crush, we soon became incredibly close friends. A couple weeks ago, we both confessed we had strong feelings for each other, and we have been out a few times without anybody knowing about it. But now we both, for lack of a better word, love each other more than anyone else either of us has dated before. We really want to step up our relationship, or at least stop having to sneak around about it.

I love my brother, and I don't want to do anything to hurt him. I know that, as things stand, if he ever found out about me and C, he would never speak to me again, if not kill himself. But at the same time I hate him for how he treats her, and how he is preventing us for even attempting at a potentially great relationship. I kind of want to go ahead, regardless of how that hurts him.

Can you condense the text into three sentences or less?
<bot>: Secretly dating my bro's ex. We both want to step up our relationship, but my bro would disown me and may kill himself.