Help your brother delete your sisters channel.
It seems to be the only way to get the message across. If he allows his own family to step over him like this he is going to be taken advantage off by everyone his whole life. He needs to learn how to draw the line.
I’m an avid gamer and what she did is sadism. It’s terrible that people treat other people’s hobbies like it’s beneath them. Call me immature and don’t take my advice but if I was your brother I would do the same thing to her with something she cares about. Buuuut your best options are to help encourage him and just be in his corner because he’s surely down. You don’t have to understand his hobby to understand caring about something. Also a note try to maybe help your parents and sister understand how fruitful gaming actually is. It’s a growing profitable industry that has a lot of positive influence on today’s culture and youth. And it’s a hobby not to be taken lightly. And if they continue to treat him like a 1/3rd of a person then he’ll free himself from them. Which I would encourage him to do anyway. He doesn’t need people who obv don’t care about what’s important to him. As long as his gaming isn’t unhealthy for him or have any negative impacts on his functionality as a day to day human being let him be himself.
Get your brother to delete her YouTube account. And her IG for good measure