Praise be to Allah.
The question mentions “the father’s wife” and later on “the mother.” As the ruling will vary, we will answer the question in such a manner as to address both possibilities.
1 – Division of the estate left by the dead son:
His two wives get one-eighth, to be divided equally between them.
His father gets one-sixth.
If the woman referred to in the question was his mother, she gets one-sixth; if she was his father’s wife, she is not his mother and does not have any share.
The rest goes to his four sons to be shared out equally amongst them.
2 – Division of the estate left by the father’s wife or the mother.
If she was the wife of the father and had no children – either from her current husband or from another if she had been married before – then her husband gets one half. The questioner has not told us about the rest of her heirs so we cannot state how the rest of the estate is to be divided.
If she had children – either from her current husband or from another – then her husband gets one-quarter. The questioner has not told us about the rest of her heirs.
If she was the mother, then her husband gets one-quarter, and her children who were alive at the time of her death get the rest, with the males taking twice the share of the females.
3 – Division of the estate left by the father.
His wife – if he had a wife – gets one-eighth.
The rest is to be given to his children who were alive at the time of his death, with each male taking twice the share of a female. If he did not have a wife, all his estate goes to his children, with each male taking twice the share of a female.
With regard to the children of the son who died before him, they do not inherit anything from their grandfather or grandmother. The fact that the son would have inherited if he were still alive does not mean that his share is given to his children if he dies, because the children in this case are prevented from inheriting by the presence of their paternal uncles. This is according to scholarly consensus.
And Allaah knows best.