I am sorry you're having to go thru this especially with your mom ill. It is interesting he is the youngest (if I understand correctly) and obviously acts as if he feels entitled. It could be that...or it could be he feels and felt left out for much of his younger life that he's reacting to that. There are lots of reasons, and I gather the emotional issues in your family run deep and long.

My only recommendation is that you take care of yourself, keep doing what you feel is right for your mom, and even tho I know it's hard, try not to let him get to you. This is evidently a pattern with him, and I'm sorry for that because I know it can't be easy for others in your family trying to help your mom.

Take care