What is the ruling regarding a plot of land that has not been built upon—should it be given to the heirs of the deceased niece's son, or should it be entirely re-divided among the original numerous heirs?

If the sisters relinquish their inheritance in the land to their brother in exchange for him raising, caring for, and marrying them off, then there is no sin upon him in taking their share and his brother's share. He may give the remaining portion to his deceased sister's son—who passed away before her father—as a charitable donation, not as an inheritance. However, if they do not relinquish their share, then it is not permissible for him or his brother to take the land; rather, it must be divided among the heirs. If the brother had spent on them with the intention of being reimbursed, then he is only entitled to take what he spent and must return the rest to their heirs.