Can I eat all of my sister's lactose-free snacks?
Why would you eat up all your sister's snacks?
I am hungry and feel like it.
Your sister would not allow you to eat all of her snacks, so you doing so is inconsiderate of her rights. Buy your own snacks to enjoy.
I see what you're saying but why wouldn't you eat all of your sister's food?
For the same reason that you wouldn't want your sister to eat all of your food, it is considered rude to take someone's food without their permission. Just ask her first if you can have some.