He died and left behind two wives, a mother and four children; how should the estate be divided?

Praise be to Allah.

If a man dies and leaves behind a mother, two wives, two daughters and two sons, then the estate is to be divided as follows: 

The share of the mother is one sixth, because Allah says (interpretation of the meaning): “For parents, a sixth share of inheritance to each if the deceased left children” [al-Nisa’ 4:11]. 

The two wives get one-eighth, which is to be divided equally between them, because Allah says (interpretation of the meaning): “their (your wives) share is a fourth if you leave no child; but if you leave a child, they get an eighth of that which you leave” [al-Nisa’ 4:12]. 

Ibn Katheer said: Regardless of whether the share is one-quarter or one-eighth, it should be shared out equally between one wife, or two wives, or three wives, or four wives.

End quote from Tafseer al-Qur’aan al-‘Azeem, 2/229 

Al-Qurtubi said: They are agreed that the share of one wife, two wives, three wives or four wives is one-quarter if he did not have children, and it is one-eighth if he did have children, and they are to share that, because Allah, may He be glorified and exalted, did not differentiate between one wife or several in this case.

End quote from al-Jaami‘ li Ahkaam al-Qur’aan, 5/75 

Whatever remains of the inheritance is to be divided among his children, both males and females, with each male getting the share of two females, because Allah says (interpretation of the meaning): “Allaah commands you as regards your children’s (inheritance): to the male, a portion equal to that of two females” [al-Nisa’ 4:11]. 

The grandchildren do not get any share of the inheritance if their father and paternal uncles are still alive. See the answer to question number 70575. 

And Allah knows best.