I feel like they were a bit harsh with the big bro there. Like yeah he isn't being accepting, but they jump immediately to "stfu and accept it, or we'll punish you!" The boy's confused because this "isn't normal" in his eyes, best remedy in this situation is to educate, not threaten. Talk to him civily, give him the chance to express what he's feeling - even if it's not nice thoughts - and talk him through why there's nothing wrong with his younger brother wanting to transition, and how hurtful his rejection can be. Being aggressive right out the gate is only going to push him further away.