Pregnant cat adopting sister's kittens

I've got 2 cats, one had kittens two weeks ago and her sister is almost ready have her own.

Now she is starting to nurse her sister's kittens... Is this normal, should I keep her away?

Cats sometimes do steal other cat' kittens. As long as the kittens are ALL nursing and staying warm, let them be. If First Cat has too many kittens and not enough nipples take some of those kittens and see if the other cat will accept them.

However, if Sister Cat seems to be grieving because First Cat stole all her babies, give some of the kittens back to Sister Cat. Just make sure they're all warm and nursing, that's the important thing.

I would separate the pregnant cat from the kittens in order to save her colostrum for her own kittens. However, it is not uncommon for sister cats to nurse each other's kittens once they are born.

You really need to make sure that all the kittens are nursing well and growing once the next litter is born. Weighing each kitten daily will tell you if any baby isn't getting enough.

Do you plan to spay the moms after their kittens wean?

Oh, wait, Sister Cat hasn't had her babies yet? Sorry, I misunderstood, I thought  all kittens were born and alll that stuff. Never mind.