What are the consequences of going to visit grandmother's house, after having sworn in a moment of anger not to go?

If the oath is merely an inner thought and not uttered, it is not considered an oath, for the Prophet, peace and blessings be upon him, said: "Indeed, Allah has forgiven my Ummah for what crosses their minds, as long as they do not act upon it or speak of it." However, if you utter the oath verbally, then this is a binding oath, and if you break it, you must offer expiation. Since the matter concerns visiting your grandmother, which is an act of maintaining kinship ties, you should go visit her, break your oath, and offer expiation for it, for the Prophet, peace and blessings be upon him, said: "Whoever takes an oath and then sees something better than it, let him do that which is better and offer expiation for his oath."