Am I sinful for not visiting my paternal grandfather's wife and for preventing my mother from visiting her due to a family dispute, and what is the expiation for this feeling of guilt?

There are two types of kinship (rahim): kinship that must be maintained and is forbidden to sever, which includes all unmarriageable kin (like grandfathers, grandmothers, paternal aunts, paternal uncles, maternal aunts, and maternal uncles); and kinship that is disliked to sever and recommended to maintain, which includes all marriageable kin (like paternal uncles' sons and maternal uncles' sons).

The wife of a grandfather is not considered among the kin of her husband's grandchildren. Maintaining ties with her is recommended, not obligatory. Therefore, the questioner did not commit a forbidden act by preventing her mother from visiting her daughter's grandfather's wife, especially since she is harmed by the visit, as visiting is not obligatory in this case.

It is recommended to seek forgiveness for the deceased and to strive to maintain ties with kin such as the grandfather, paternal uncles, and paternal aunts, and to endeavor to reconcile disputes.