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, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
Whoever dies and leaves behind a mother, a wife, two daughters and a brother – a full brother or from the father’s side only – then his mother and brother have the right in the inheritance. In this case, his mother would get one-sixth of the inheritance and his brother the remainder after the share of the wife, mother and two daughters. However, if he is a brother from the mother’s side only, then he would have no share in the inheritance because of being prevented by the existence of the daughters. His mother or his brother - who is entitled to the inheritance - have not the right to ask for the whole inheritance; rather, they are only entitled to their legal share.
The estate of the deceased is every possession that he owned when he was alive and left behind, including his legal share from the inheritance of his grandfathers whether he had already received it or died before receiving it.
As regards the fostering of the two daughters, then the right of fostering is for their mother unless there is an impediment which prevents her from the fostering, like becoming a slave, apostating (from Islam), or becoming a dissolute person, or getting married to a non-Mahram (i.e., marriageable man) to the daughters, in which case, the right of fostering is transferred from her to her own mother, but not to the mother of her husband, according to the view of the majority of the scholars .
This is in regard to fostering. For more details, please refer to Fatwa 84618.
As regards taking care of the wealth of the two daughters, it is not for their mother to do so; rather, it is for the person who was appointed by their father to be their custodian. In case he did not appoint a custodian over them, then it is their parental grandfather and then his (the daughters’ father’s) brother who should take care of their wealth. Please note that the scholars have given further details about this.
Finally, we advise you first to fear Allaah and then to not resort to man-made laws (i.e. civil courts); rather, you should take the case to the trustworthy scholars in your country. However, in case you are forced to take your case to those courts, you would not be sinful in doing so.
Allaah Knows best.