Me again sorry. This is bugging me for some reason!
If your mum is the one who's worried about you not eating, why is she not more interested in helping you to solve the problem? Have you asked her for help?
I understand why you have to prepare your own lunch, but why do you have to buy it as well? Presumably food is provided for your brother if he has no money, why does yours have to be separate? I think if my daughter still lives here when she's old enough to start earning her own money, I'll find it easier just to ask her for a contribution towards general household food and bills rather than making her buy her own. Then we'll just carry on as we are now - one person cook a main meal for everyone, and fill the fridge and cupboards with a general selection of other foods for lunches, and everyone helps themselves with no need to mark anything as reserved for any one person. My daughter is only 11 but already helps herself to her tea and is getting pretty good at working out the carbs, she doesn't need much help from me any more; and if I've forgotten to buy whatever her favourite fad is that week, then she just chooses something else. There's always basics like bread, eggs, cheese etc that you can do something with.
Sorry if this sounds a bit rude, it's not meant to; I'm just trying to understand why you've got a problem in the first place. Surely the food you buy can't be the only food in the house? So if "your" food disappears there must be something else you can eat instead? Unless you have any other special dietary requirements which you haven't mentioned, but type 1 diabetes alone does not require special food.