You should both get the same. The fact your brother will own the house and annex does not make it any fairer even if he has to tolerate looking after the father from age 60 to 100.
I am making sure all my children have exactly the same amount towards housing, no differences, no exceptions, no matter what they earn, even if they win 100m on the lottery or earn 10x what another of them earns, they will all get the same.