Is she greatly sinful for not visiting her deceased father-in-law, and for depriving her children of visiting him, due to the problems that existed between them?

A wife is not obligated to visit her husband's father. However, it is not permissible to deprive the grandfather of seeing his grandchildren, as this constitutes a severing of kinship ties. The wife should repent and seek forgiveness, and strive to show kindness to the deceased through supplication or charity.