When a person makes a joint Will with another person it is like an agreement between these two persons as to how their estate should be dealt with post their demise
Thus if one testator dies prior to the other testator then the living testator is bound by his agreement and he cannot change the distribution of the estate as directed in the joint Will by making his own Will which is in variance to the directions given in the joint Will, for, by doing so, the living testator would be stultifying the wishes of the deceased testator
So in my view the Joint Will would prevail over the subsequent Will by which the father's 1/3 share in the mother's portion came to be bequeathed to your brother which is not what your mother wished and desired in the joint Will. Thus your father could not have dealt with your mother's 50% share in the way he did and thereby given a complete go by to the wishes of your mother
Yes unless ofcourse the joint Will is disproved by your brother or any other legal heirs or when you fail to prove the joint Will, the father's subsequent Will which is in derogation of the Joint Will would prevail
Thus in my view the entire estate should go to the legatee named in the Joint Will