Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that a man said: O Messenger of Allah! My sister vowed to perform Hajj and she died without having performed it. He (peace and blessings be upon him) said: "Tell me, if your sister had owed a debt, would you have paid it?" He said: Yes. He (peace and blessings be upon him) said: "Then the debt of Allah is more entitled to be paid; perform Hajj on behalf of your sister."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12390