Sometimes people really just prefer to make their own food, and can be picky about it. I know I am that way and it frustrates a lot of people around me. I will be more than willing to pick up some goodies for my team-mates, and cook for others. If someone wants to offer me something, or cook for me I usually refuse. If I wasn't part of the process making it, I really don't want to eat it. It's to the point I rarely go out to eat because I really detest not being part of the process of making what I am eating. I don't know if it is similar for your sister, but sometimes people can just be very picky.