When my brother was ill, I prayed for his recovery and "promised" to fast two months if he did so. After two months he did. However, I am a diabetic with a heart problem and I receive insulin twice a day. So I have not fasted since his recovery. What can I do?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is obligatory on everyone to fulfill his vow. Fulfilling a vow is among the qualities of the pious people, who will enter Paradise, Insha Allah. Allah Says (interpretation of meaning): {Then let them complete the prescribed duties (Manâsik of Hajj) for them, and perform their vows, …}[22:29] He also Says: {They (are those who) fulfill (their) vows, …}[76:7].
The Prophet said: "Whoever vows that he will be obedient to Allah should remain obedient to him; and whoever made a vow that he will disobey Allah, should not disobey Him". [Reported by Imam Bukhari]
Therefore, who has vowed to obey Allah should fulfill his vow; otherwise he has to pay an expiation of breaking an oath. The expiation is to feed ten poor persons or to clothe them or to free a slave. But, whoever can not afford that, then he should fast for three days. The Prophet said: "Whoever takes a vow on himself and he can not fulfill it, the atonement for it, is the same as for an oath". [Reported in four books of Sunan].
Thus, if you are able to fast or hope that you will be able to fast in future then it is not permitted for you to turn away from doing so since the issue of making a vow is very sacred in Islam.
However, if you can never fulfill your vow you have to expiate instead of fasting as mentioned above.
Allah knows best.