Stepmom Bans Me from Eating Her Food, Then Expects Me to Cook for Her Guests. AITA?

A 16-year-old boy has taken to Reddit's Am I the A**hole forum to ask whether he was in the wrong for refusing to cook dinner for his stepmother's guests. The teenager's stepmother had previously refused to let him eat any food she cooked, and he had been cooking his own meals. However, when his stepmother asked him to cook for her sister's children, who were visiting after an accident, he refused. The boy's father later grounded him, telling him he should have done the favor for his stepmother. But the boy argues that his relationship with his stepmother is already beyond repair, and he didn't want to do anything to help her. What do you think? Was the boy justified in his actions, or should he have put aside his differences with his stepmother to help out in an emergency?