Must I give my sisters a share of the gift that my grandfather bequeathed to us—my brother and I—from his land, to the exclusion of my sisters, so that my grandfather is not held accountable?

The relied-upon fatwa is that justice among grandchildren is not obligatory. However, your sharing with your sisters from your share is a righteous deed and a precaution to absolve yourself and your grandfather, according to the other opinion.