Dad and son were partners in the corporation.  Let's say they were 50:50 ownership.  OP does not spell that out, but, for the sake of argument, let's say that is true.

So, when Dad dies, Dad owns 50% and Son owns 50%.  So, when dividing up the estate, Dad's 50% would be split between Son and Daughter.  So, at most daughter would get 25% , not 50%.    Agree?