A Conditional Vow

Question :

I vowed to fast a day for my sick brother, then circumstances prevented that, until my brother died. Should I fast this day after his death? And should I fast one day or two days? Also, I visited the grave of my deceased brother twice, does Islam allow me to do this? And is it true that the dead are aware of the visits of the living to them?

Answer:

If your vow was to fast for a day, on condition that Allah cured your sick brother, but he died without being cured, there is no need for you to fast it and this is what is apparent from the questioner's query. It is permissible for you to visit the grave of your deceased brother once, twice or more. This does not mean that you visit him always, because this is not lawful, but you may visit him and supplicate Allah for him occasionally.

As for him knowing who visits him, it has been reported from the Prophet ﷺ that:
"There is no Muslim man who delivers salutations to his deceased brother who knew him in the life of this world, except that Allah returns his soul to him and he answers the salutations."

This Hadith was authenticated by Ibn 'Abdul-Barr and Ibn Al- Qayyim transmitted it from him in his book: Ar-Ruh and he confirmed it.

But you must know that visiting the graves is for the benefit of the dead, and for the living to take warning, and not so that a person may supplicate to the dead. This is because supplicating to other than Allah is major Shirk which removes a person from the pale of Islam. The purpose of it is not for a person to be intent on supplicating next to them either, because supplication in the mosque is better than supplication there; indeed, making the intention to supplicate at the graves is an innovation and it could even lead to Shirk.

Source:
Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 6 Pages 422-423