Hi Kyle, welcome to Psych Central. I can see how this is a challenge for you. It sounds like your sister feels entitled to take your things because she helps you with your car payments. It can be tricky when someone helps you like that if you need the help and yet they begin to think it's ok to invade your boundaries. Let that be a life lesson for you where you work on being more independent so you don't end up dealing with dynamic.

If you say something about this problem, you can end up not only paying for your own coffee but also for your car loan too. The only other thing I can think of is hiding your coffee in your room so she can't get to it. Yet, she will figure that out when you make coffee for yourself when the coffee runs out in the cabinet.

A lot depends on her personality too. Sometimes you can sit and talk about how you struggle to keep your coffee and milk supply and you do appreciate that she helps with your car payment, but you are a student and stuggle to afford the coffee and milk you buy for yourself too. Sometimes a person will be kind and see how you struggle and yet other times a person may take offense and throw what they do for you and that you should not complain too. Only you know your sister's nature and how she may react if you talk to her "calmly".