My sister is going away with a group of friends she hasn't known very long - they're going to a self catering cottage for 4 days. She's on a diet and is having a panic about the rest of the group wanting to food shop as a group and all cook and eat meals together. She doesn't want to make a fuss or look like she's being difficult. I told her to stop being a wuss, man up and just tell them she'll bring her own food.
What's the etiquette with this sort of situation? Is it unreasonable to want to bring your own food or will this look like she's being difficult?