Is this normal or is she just acting stupid?

.....Yep, she needs to grow up and, no, this is not normal. She does not have the right to lay claim to food that is intended for the entire family. She's being very selfish. Next time she says, "They're mine," just let her know right to her face that food in your house belongs to the whole family and she does not and will not have the right to claim it as her personal property. You should talk to your parents about this and they should make her stop it. :-)