I think you are making this too complicated.
While it's great that the siblings all work together, it's unlikely that a larger(?) group of cousins would be able to share ownership of a house without fighting.
Although the siblings get on well, when they die, their surviving spouses might not.
The siblings could give the brother a lease with a nominal rent for the rest of his life.
When the brother dies, the property should be sold and the proceeds distributed to the siblings. They can do what they like with their share after that.
What happens if the brother moves out? What happens if the brother needs long-term medical care? You may wish to sell the house at that stage.
If the siblings don't want to benefit, they should consider passing up on their legacy or selling the house to the brother. If the brother is not competent, the siblings might consider selling the house cheaply to one of the other siblings and trust that he will sell it and distribute the proceeds to the nephews and nieces. CAT issues might arise here though. A parent can leave a child around €225k to a child tax free but only €30k to a niece or nephew.
Brendan