What is the ruling on swearing and taking an oath by Allah, as in the the statement: "By Allah, my grandmother will not die until she attends my wedding"?

There is no harm in swearing by Allah for good, and we have explained this previously. Sheikh Ibn Uthaymeen stated that swearing by Allah is of two types:

1. That which is caused by the swearer's strong trust in Allah, while acknowledging his own weakness. This is permissible, and its evidence is the saying of the Prophet (peace and blessings be upon him): «Many a disheveled, dust-covered person, driven away from doors, if he were to swear by Allah, Allah would fulfill his oath», and the narration of Anas ibn An-Nadr.

2. That which is caused by arrogance and self-admiration. This is forbidden and may invalidate one's deeds. An example of this is the story of the man who swore that Allah would not forgive a certain person, so Allah invalidated his deeds.