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.
We are not aware of the Hindu law and we do not know whether or not it states that your son-in-law has the right to inherit. In any case, as Muslims, we resort in our cases to the Islamic law and not to the man-made law. The son of your sister has no right to inherit his grandfather from the mother’s side because the son of the daughter is not among the heirs but he is a relative from the female side of the deceased. Therefore, the son of your sister has no right in the inheritance of his grandfather [the father of his mother] whether or not there is a communication between you and him.
Moreover, if we presume that the man-made law gave him a portion of the inheritance, then it is not permissible for him to take it without the consent of all the heirs. So, what should be taken into account is what the Sharee’ah says and not what the law that contradicts the Sharee’ah says.
Allaah Knows best.