Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Rulings concerning inheritance are so numerous that they cannot be stated in one Fatwa. So, anyone who wants to learn such rulings should study them from Sheikhs and knowledgeable people. To summarize people who inherit are of two kinds: (1) Those who have a specified share (Furud). These are: the husband, the wife, the maternal brothers, the daughters, the grand daughters (from the son’s side), the full sisters, or paternal sisters, the mother and the grandmothers or great-grandmothers.
(2) Assaba (blood relations); these include the sons, the grandsons (from the son’s side) the fathers, the grandfathers, the full brothers or paternal brothers, the nephews from the brother’s side, the paternal uncles and the paternal uncle’s sons. Some of these also have specific share (Fard) like the father and the grandfather.
As for the inheritance in question, if there is any way of differentiating what each of the two owned that would be better. In such case, the husband’s sister takes half of his wealth if she is his full sister or his paternal one, otherwise she (maternal sister) takes the sixth. What remains is distributed among the children of the brother if he is a full brother or a paternal one. But if he is a maternal brother his children have no share from the inheritance, the same rule applies to the children of the sister. In this case, the remaining inheritance is given to further relatives from the paternal side like the uncles or their children if they exist otherwise what remains is returned to the sister. Then the wealth of the wife is divided in shares among her inheritors if she has any otherwise, it is put in the Muslim treasury or in their public interests.
Now, if there is no way of knowing what each of them owned, then the inheritance is divided in equal shares among their inheritors.
Finally, know that matters related to inheritance cannot be determined by a simple Fatwa. Indeed, its distribution should be done only by a Shariah court otherwise a trustworthy Islamic Center. This is so because such a matter needs a deep insight into all related matters.
Allah knows best.