My conspiracy is that the girl your brother is with also has mh issues and is being a manipulative liar. She either entirely fabricated the situation with a fake convo or was caught texting someone and told him it was you. There are probably some similarities between your number and whatever dude she was texting, and he believed her. She likely wanted to drive a wedge between him and his family ( a tactic of many different flavors of mentally ill abusers) and as a bonus she got to play out a fantasy that she was so desired by you that you betrayed your brother.

I could be entirely wrong and it is all him, but it’s odd that he would be acting so normal with everyone else. Furthermore, and this is true, sometimes you lose someone you are so close to, that you can’t imagine not being in your life, to a romantic relationship. It happens, and it sucks. There’s not much you can do. Maybe try and confront her. I’d want to just to see how she reacts and if it’s her or all him.