What is the religious ruling on mistreating one's grandmother, and is there anything mentioned that encourages good treatment of grandparents or clarifies their rights?

The grandmother has a great right over her grandchild, and she is among the foremost people to be honored, after the parents. Many scholars have gone to the extent of stating the obligation of honoring the grandfather. In fact, Ibn Hazm explicitly stated the consensus of scholars on the obligation of honoring the grandfather, and while this consensus was disputed, the majority of scholars hold this view. In any case, the grandmother is among the kin (arham) whom Allah has commanded us to maintain ties with and treat with kindness, and forbidden us from severing ties with. Allah Almighty says: "So would you, perhaps, if you turned away, cause corruption in the land and sever your ties of kinship? Those are the ones whom Allah has cursed, so He made them deaf and blinded their vision." Ill-treatment is absolutely impermissible, but it is even more severe when directed towards a grandmother and close relatives.