I have to choose between family members....I'm not sure what to do?

First of all, your mom and your aunts have no say so in it. He's a grown man who has obviously made a pretty good life for himself as well as them. This could be his way of grieving and he cannot and must not be condemned for it. It kind of seems that they are afraid he will go through all of his money and there will be nothing left for them when he dies, otherwise they would stand behind him no matter what. He is not disrespecting your grandmother in anyway. It's simply his way of dealing with his loss. The sooner you mom and aunts realize that, the sooner peace comes back to the family. Anyway, they don't have to approve or agree with what he is doing. They just need to love their father regardless.