I have two brothers. My younger brother is pretty cool - always kind and talkative, even though our interests don't overlap much. He's like an extroverted friend who always engaged with you, even if you don't have much interesting to respond with.
My older brother and I barely know each other by now, but he seems to have invented a fictional version of me who he absolutely despises. Every time I see him, he gets quieter and quieter, and he regularly texts our mother with the most venomous insults and accusations he can sling my way. I've probably spoken fifty words to him in the last few years, but he's fully convinced that I'm an evil person working to slight him at every turn.
I wonder sometimes if I should talk to him directly and ask what's up, or if I should just write him off. Even my mother is confused as to why he's like this.