What is the religious view of a deceased man's sisters who covet his estate, accuse his wife, and try to take guardianship of their brother's children away from her?

It is forbidden for the husband's sisters to take anything from the husband's money that he entrusted to them, if that money rightfully belongs to the wife or her children. This is considered consuming wealth unjustly. The wife has the right to present the matter to the court to recover the money that was taken wrongfully. As for guardianship over the minor children's wealth, after the husband's death, it transfers to his appointed guardian, then to the judge or whomever the judge appoints. It is advised to raise the matter to the court to demand the appointment of a trustworthy guardian for the children's wealth.