Dealing with a negative family member…or a person who just likes to put you down
you mentioned that he didn't put you down when you were talking privately but did so in the group chat. that's behavior of an underlying issue. he probably has feelings of inadequacy or esteem issues and running you down in front of other people make him feel better because he thinks belittling you empowers him.
sounds like a real dildo.
anyway, you can broach the subject to him privately and tell him that you don't like that and it hurts your feelings when he does it and you'd like it if he didn't do it any longer; and ask him if there's anything you can do to help him with his issues. and he'll probably blow you off like the fragile, mentally unstable person he is.
so that means you get to put him on blast. but you don't do that privately, you do that full tilt under the lights of the big stage. like at family dinners, or in the group chats. when he tries some sideways **** of talking you down, then you hit him with both barrels in front of everybody: bro, i know that you have feelings of inadequacy and low self esteem and the only reason you're saying these things is that you think by making me look bad it'll boost your self worth. playing the hater and running me down isn't going to fix your own issues of self doubt, lacking personality and meager confidence. but nice try, man.
or, you can kick him in the sack and tell him to knock it off.