I don't think my sister is raising her children right, what should I do?

First let me say I'm nineteen and I have no children and my sister is twenty-five and has two children, every time I've over there I can't help but feel like she's raising them all wrong. They have no real bedtime, they have no regular meal times and what they do eat is mostly junk food and sugary candy, they never have milk in the fridge and a lot of the time there fridge and cupboards are almost completely empty, they don't bathe regularly (her son has eczema and he's not suppose to have a bath everyday but she still doesn't seem to do it often enough), her daughter has been wearing the same outfit for at least a week and she never has underwear on, they never have clean clothes, she doesn't make either of them wipe after going pee or wash there hands, she let's them do whatever they want and will only yell at them to stop but not make any effort to actually make them stop, they hit each other and cry when they don't get there own way. My other question is when I'm over there, I make a point to try and keep them in line and when I say something I go through with it unless either parent says otherwise and then I make a point to let the child know I'm not giving in, I was just given other directions. Anyway, the other day I was making them go to bed, every time the boy got out I got up and put him back and gave him a quick smack, I honestly had no issue with doing this and the girl was being good but the boy was using every excuse he could think of, he was thirsty, he had to pee, he was hungry, he wanted his Mom and Dad, and then after probably half an hour my sister just brought up some cereal and a drink and I'm thinking you know he's lying and I just spent all this time for you to just give in and eventually he came downstairs once his sister was asleep, am I wrong for being upset because she always gives in even when I try and do what she wants but won't and am I being unrealistic because I have no children of my own and probably don't see how hard it really is. Also, I might start babysitting them two days a week, probably eight hours a day and how do I tell her I have no problem doing it but I want them to understand that they (the children) have to do what I say and that they (the parents) can't override me at least while I'm babysitting or again is that unreasonable.