Should they be eating my food?

So I am a broke college student living with a relative in the house that my grandparents own. My relative is extremely unhealthy and doesn't ever buy any groceries and when they do it's mostly junk snacks or ready to make freezer meals. I have problems with my stomach/ gastrointestinal system so I can't eat that type of food, but sometimes I do because I can't afford to do anything else. I have started purchasing my own healthy groceries specifically for when I'm on the go for work or school & so I can maintain my health. I spend a lot of money that I don't have buying this food and need it to last for the rest of the month. I came home to 
 Discover a considerable dent has has been  made out of certain things. They will only do this when I'm not here and are extremely sneaky about it but it is just us two in the house so I know it's them. Anyways, I just don't know what to do about this. To some it may seem like no big deal but I am not a wealthy by any means and it is when you barely make enough for basic needs 

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