All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
As regards how to distribute the inheritance of your brother, if he died leaving behind a wife, two sons, a father, a mother, brothers and sons, then each of his father and his mother gets one-sixth of the inheritance, as Allaah Says (what means): {And for one’s parents, to each one of them is a sixth of his estate if he left children.}[Quran 4:11] His wife gets one-eighth as Allaah Says about the shares of the wives (what means): {But if you leave a child, then for them (i.e. the wives) is an eighth of what you leave, after any bequest you [may have] made or debt.}[Quran 4:12] The remaining of the inheritance should be equally divided between his two sons as the Prophet said: "Give the prescribed share of inheritance to those who are liable for it and the remainder [of it] is for the nearest male blood relative." [Al-Bukhari and Muslim] However, the brothers and sisters have no right in the inheritance because they are prevented from it with the existence of the sons and the father.
Therefore, the inheritance should be divided into 48 shares; the mother gets one-sixth, which 8 shares, the father gets one-sixth, which is 8 shares as well, the wife gets one-eighth, which is 6 shares, and each son gets 13 shares.
However, the wife is not permitted to ask for the share of her husband from the inheritance of his father who is still alive; rather, even if the father died, his son who had died before him has no right in the inheritance. Therefore, she is obliged to give back the properties of her husband that she had taken because they are part of the inheritance which is the right of all the heirs, each according to his legal share.
Nonetheless, the wife has the right of fostering her two sons, but she has no authority over their wealth, so the share of the sons is not given to her unless their father had made a will to this effect. If there is no will, then it is their grandfather –the father of their father –who becomes the guardian of their wealth, and if they have no grandfather, then it is the Muslim judge (who should be the guardian of their wealth).
Besides, she is not permitted to prevent the family of her deceased husband from seeing or taking her sons during the day, but they should spend the night with her only. Finally, if you notice that the wife of your brother is obstinate (in her error), then we advise you to take the matter to an Islamic court in your country so that it would look into the matter and give each one his right.
Allaah Knows best.