What is obligatory to do regarding a sister who does not visit her mother after being advised, knowing that she claims there is nothing on her mind but she does not want to visit anyone, and that the mother is fifteen years older than her?

Visiting one's stepmother and showing her kindness is an act of filial piety towards one's father, whether he is alive or deceased. Therefore, the sister should be kind to her stepmother and visit her, knowing that this is part of her devotion to her father. If the sister does not respond to her visit, it is not permissible to boycott her or treat her poorly, because she is not obligated to visit her stepmother, especially if her reason is a general disinclination to visit rather than a deliberate severance of ties.