OP again.
I could’ve written this. I have an 18 yo dd and 14 yo ds. He tells her she smells bad and is fat. He refuses to interact with her. It’s bad.
Sorry to hear that, PP. I hope that some of the advice in this thread is helpful to you.
My younger brother (4 years apart) became very cruel when he became a teenager. I think my parents and even I never did anything because he was the "baby" and we felt bad for him. These comments seemed to come from a place of insecurity. But it really hurt that my parents never said anything. This went on until he was 30 and I realized he wasn't going to "grow out of it". We have no relationship now and I blame my parents for favoring him.
Sorry to hear about your experience, PP. I wonder if there are issues, other than being a teen, that caused him to act like that until age 30? That's absolutely not an excuse for his behavior, but it may explain it.
OP it is good you are tuned in and care. One way sibling rivalry ran on both sides of my family and nobody did anything to stop it. My older sister was downright obsessed with competing with me and trying to destroy me emotionally. It was NOT normal. In her case I suspect it was both signs of depression and a personality disorder (not saying this is the case with your son). They did little to discourage her cruelty. She has since struggled her whole life to have normal relationships. She has few friends, never has been able to have a healthy relationship with a significant other and can only be a decent person who others like when on anti-depressants. I can tell when she goes off them, because she becomes nasty again.
Sorry to hear about your experience, PP.