I don't think there's actually anything grammatically wrong with "Please serve foods/meals," in a general sense. The "problem" is mostly because of English language "culture" (specifically, the Gricean Maxims).
It is maybe more correct to say, "Please serve them food" ("food" is usually an uncountable or "mass" noun. "foods" means, simply put, "2 or more types of food."). Or, "Please serve them meals/a meal."
The way you are using these sentences may be "wrong," though. We often use "Please serve them food/meals" only in a general way. For example, "How can I help you with the guests?" "Please serve them food (during their stay)." Or maybe they arrived between lunch and dinner, so in a culture without tea time, you might give them sweets or a small meal. For example, "I will talk with them for a long time about this thing, so please serve/prepare them food (for me)."
However, if the guests arrive for dinner, then that meal has a specific name, so we will prefer to use the name. We don't say "food" or "a meal"/"meals" when we can be more specific and informative. If your guests arrive at dinnertime, say, "Please serve them dinner (too)." It's the same for lunchtime or breakfast time.
Another way to ask your sister to prepare food is, "Please bring them something to eat/drink." Or, "Please prepare tea/a snack/something for them to eat."