It was explicitly stated that he knew what‘s up. Dunno what you want him to mind read or what the point of your post is. Dude is her family and could‘ve helped but didn‘t, it really is that simple.
Which is okay but that makes his involvement now literally 0 impact.
I don‘t really smell drama since he is a non factor.
He pretty much isn‘t even a background character and will probably be overprotective now (which would be pretty cringe with context and I hope both mains call him out) or thank mc since he himself was too big of a pussy.
I get the general idea of not being able to read minds and preferring someone asking for help when they need help, but that’s not relevant here because the cousin explicitly established that 1. He knew about the bullying, and 2. He willingly ignored it and left her to the sharks (heh)
1. It's never that simple,knowing someone has made a change due to bullying doesn't mean one acts on it,for equal amounts it can lead to being pushed away from the victim since they already chose a solution that worked,all this also hinges on when the cousin knew or if he just infered it from the behaviour change.
2. What help could a fellow inexperienced kid even provide?,our very own main protagonist didn't even have plan n' basically dragged the shark out to face the problem herself,which was solved more by the two fellas with the bully.
3. We have yet to hear the exact arrangement the cousin has,a sibling would have it hard enough to solve an issue whilst living with the victim,let alone a cousin who might not even be around all the time to solve things,and we don't even know what his own upbringing entails.
4. Plenty of folks,if a peaceful enough staus quo(more so in the conservativeness of Japan)occurs,won't try messing any further unless told directly,if the victim themselves wouldn't even get their parents to do it,then what hope does a further relative have in prying?
People be bashin' him with barely any info and even demanding he leave after merely TALKING to the main protagonist.
She didn’t ask either this time isn’t it ? 
