Is it rude to have your guest buy groceries?

My brother his wife and 2 kids are staying a week. (Self invited) we normally can have groceries last us about 2-3 weeks when we go shopping. My brother eats so much. Today for dinner he had our leftovers as well as dinner I made and some snacks. So it would mostly be for him to eat! Would you be offended if you were asked to buy food!?

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