Honor Dad's wishes!  Don't lose your family over the money.  If the son does give more, he will resent the sisters forever even if he says he won't.  It will always be there somewhere deep inside.   It sucks for the sisters, but as stated, who's money was it to give?  Some how all the family must except Dad's plan or you will all lose your family over money.

It always saddens me when a parent has something when they die, it can ruin thier family.  If he was broke, the family would happy and intact.  Kind of sickening to think about. 

To anyone out there with assets and a will, PLEASE tell you children what the will says while you and your spouse are still alive.  That way you children can ask you why and get used to the idea.