If my mother even raises her voice at anyone he will fly onto her head & start pecking her. I have to admit the first week I couldn't stop laughing long enough to discipline. Once in the kitchen my mother grabbed a metal mixing bowl & shoved it on her head as a helmet then hid under a blanket
I'm afraid you put on quite a show for your bird, he probably really enjoyed it. The best response is to calmly (as possible) remove him from your mom, tell him no & everybody leave the room. The aggression is a separate issue, your mom should work w/him alone in a positive manner. She can sing to him, give him treats, or just spend time w/him. He is just protecting his flock, so she may want to make sure he is caged or in another room b4 she gives you guys a talking to.
I have similar problems w/Merlin & he will never feel the same about my husband as me, but he's definitely not allowed to attack him. But it happens, I don't laugh (sometimes when I'm mad, I'm tempted). We do timeout in the cage or my bedroom. He loves any kind of attention, even negative attention. He's a drama king.. Birds aren't dogs & I don't think they can be trained to do anything 100%. Merlin knows when he does something wrong, he will look @ me as if to challenge whether I'm going to do anything. I try to respond the same way everytime, even when its not convenient.
Good luck & its definitely worth it.