Charging an 8 year old for food? Update

My sister's daughter's friend is staying with them for a little while because of stuff with her mother. The little girl is 8. My sister took the time to write prices on all the food in the house. The girls father sends her $20 a week, and my sister makes her use the money her dad gives her to pay for food. If she wants an actual meal like dinner she has to pay $10. Otherwise she has to find something cheaper in the house to pay my sister so she will pay for noodles. My sister makes the noodles $2 even though in the store they are like 50 cents. I told my sister that girl is 8. She shouldn't be paying her for food. But my sister said the girl isn't her kid so she needs to pay for food and thats the real world. I feel like this is wrong.

I'm glad I'm not the only one who feels this is wrong. Feel bad for the little girl. According to my sister her dad is very mentally ill so he can't care for her properly. I would take her in but her mom doesn't know me so I doubt she will let her daughter go with a stranger.

Update: I ended up messaging the mom on Facebook to let her know what was going on. Well the little girls mom is confused because apparently she was sending my sister $400 a month to care for her and the girls father was sending $200 out of his disability along with the $20 he gives his daughter. She just said thank you for telling her and idk whats gonna happen right now. So its clear my sister has money for the girl. The whole situation is making me sick and I can't look at my sister the same. I've been noticing my niece has gotten a lot of new stuff lately. I want to give my sister the benefit of a doubt that she wasn't using the money the parents had given her for my nieces new tablet and stuff. I really pray my sister wouldn't do something like that... I'm not sure what the mom is gonna do. She just said thank you to me..