I deduced from your post that you live with your father, right? Does your younger brother think of the house you share as "Missy's" or "Dad's?" I think about the way I act in my mother's house vs. my sister's - @ my mother's, I'll serve myself from the fridge. That's the house I grew up in. At my sister's, I'd probably ask, though I have to tell you, a box of cereal and some granola bars = you did well. You should go to church and say thank you that your guests only put you out fruit and cheap carbos.
6:30 AM Saturday supermarket shopping - with all due respect, if I'm your guest, I'm staying in bed, have a great Saturday, thank you.
Can I take another wild guess = you don't like your younger brother much, and you like his wife even less, and your indignance with the food is related to that. If my brother in law left the bath mat in the shower, I think I'd call and tell him how cute he was (ha! Did he not know?) I've had a guest who flooded the bathroom because he didn't realize that the curtain liner needed to go WITHIN the tub. He grew up in and owns a house with shower doors - he never even gave it a thought.
In the grand scale of things, the bath mat sounds minor, and if I caught wind of the fact that my sister-in-law and father-in-law were making snide comments because I screwed something up involving the bath mat, I probably wouldn't like them very much. But I would appreciate their sharing their Honey Bunches of Oats.