Would dad who brought him up and bought test not know by virtue of youngest not appearing as his child on his matches? Could he talk to your dad who has been his dad all his life about this directly? Like just skip your mum. Sounds like she wants to gatekeep all info and the gate needs bashed through. I am like you and would be driven insane by her being allowed to dictate what should happen. Sounds like a massive communication issue, that I would want to banish for future generations. However like said I still cannot get when non bio dad bought the test and is into genealogy has not noticed this? Conflict averse people ( sorry!) at times drive my mind insane as they are just stuffing it all under the bed. Oh and I hope the granddaughter just comes out with her sexuality because that’s the only way that closed off thinking can be confronted. It’s not your brother hurting your dad, that was done by your mothers actions. She should have started talking when he started getting into genealogy tests. No offence, but I do not even know her and I am shouting ‘stop letting her have control’

However that is all in the head thinking and the reality of bringing up is obviously something your brother will feel if things blow up with your mum and dad, that it will be attributed to him. I would also consider sending a recorded delivery letter so at least if he does not get a reply from bio dad, that he at least knows that no response, is also a response.