Vowing to visit the Prophet's Mosque if she is cured

Q: My sister became ill, so my paternal grandmother vowed that if she recovered, she would take her to visit Al-Masjid Al-Nabawy Al-Sharif (the Prophet’s Mosque in Madinah). However, my grandmother died before fulfilling her vow (may Allah be merciful to her). Should I take my sister to visit Al-Masjid Al-Nabawy Al-Sharif to fulfill my late grandmother’s vow, In sha’a-Allah (if Allah wills)? If this is not permissible, what should I do? Please advise concerning this. As-salamu ‘alaykum warahmatullah wabarakatuh (May Allah’s Peace, Mercy, and Blessings be upon you).

A: There is nothing wrong with taking your sister to visit Al-Masjid Al-Nabawy after having recovered to fulfill your grandmother’s vow. It is a vow to do an act of obedience and the Prophet (peace be upon him) is authentically reported to have said: A person who vows to obey Allah should obey Him (i.e. Should fulfill the vow). May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family and Companions.