I would likewise be devastated in your position Solar.
I like to be informed, so I would probably get some legal advice on what might happen in X, Y or Z scenario. Just so that I would know.
I think I would then carefully craft a joint letter with my sibling, with every word checked for hidden meaning to the nth degree, and give it to him.
Telling him your story, essentially. Absolutely not a guilt trip - that wouldn't be the intent.
But you know that you're unable to say these things to him, so write to him.
You have as much right to make your feelings known about your childhood home and the loss of your DM, as his new girlfriend does about what she would like to do with your family house.
Will a letter make any difference? Will it change his mind? Probably not. But at least he will be going into his decision under no illusions as to your position. And he will be at leisure to ponder on it for many years to come.
My DF made a friend several years after DM died. His friend, M, never moved into his house, was thoughtful and considerate in all of her dealings with DB and I, and took a huge burden of care from us before he died. He of course, left our family house to us. She has her house, as she didn't give it up. That's how it should be, but we couldn't be more grateful to M for everything - all her dealings with our DF. She and I are still very close, and always will be.