Six to eight weeks is when kittens can be removed from mama, I have 2 who I am trying to find homes for know, we have a lot of strays who find there way to our house....lucky us. I personaly like to wait until about 8-10 weeks, depends on the kittens and thier momma's, If they can eat and drink on their own then they will be fine. One of the other moms mentioned about the pawing as a sign of to early and she is right, but it isn't a detrimental thing, they will do fine, they will just be "pawers" which can be annoying. Our oldest cat was found at 4 weeks, momma abandoned it, and she paws horribly, but is a great cat. So I think your sister kittens are good and will do fine.
I just read some of the other response, milk is not good for cats or kittens, I can't rememeber why, just that my vet said no milk, period, they do have a kitten milk replacement that can be bought just about anywhere, and isn't to bad price wise if you are real worried.