This sounds really dodgy.
The studio that is being built, is it going to be big enough, will it be completely self contained or shared areas in the house - he’s got a few kids so that will be a busy household, and open to arguments and stress. What happens if there is a fall out, or the care requirements become unmanageable for Db and wife, and like others say, what if Db and wife split. Then there is a BIG problem.
If his property is so big, that it can have a separate studio, maybe he should sell up, move to a smaller property and pay off some debt.
There is also the situation of the debt, how much is it going to swallow up, and how can anyone be sure he won’t take more debt out in the future, secure it against the house and then risk losing a house that homes 7/8 people?
Will the money pay off his mortgage? Or will there be additional borrowing as what happens then if his rate goes up? Or the project costs go up.
As others have said, it is also risky from a care perspective. They are potentially risking their own care and future.
There are ways around clearing the debt, he can talk to a debt charity and get a DMP, which generally gives you manageable payments and 0% interest and won’t risk his house - and then he is taking responsibility for his debt rather than risking the rest of his parents future not to mention the hurt he will cause amongst the siblings and their parents.
I also think it’s really sad your parents would consider doing this; and perhaps if you talked to them about your concerns and hurt you feel they might see it from a different angle. It’s possible that they have been sold a dream by your brother that is dressed up as he’s being a selfish toad and they haven’t thought about other angles