Sorry to hear about the pregnant kitten. I think you should notify the shelter about this situation, they are partly responsible for this mess. If the their vet found that she was already pregnant before the spay surgery, why was your sister not notified of this information before she brought home the kitten? I think a lot of shelters have become too irresponsible, and could write a book about the horrors I have endured because of incompetent shelters. Please find out what your sister's rights may be in this situation, if it were me, I would demand that they handle the adoption of the kittens, and any related vet bills that your sister may, and probably will, incur due to complications in the birth, and the cost of spaying the cat after. If she decides not to allow the birth to take place they should definately cover the cost of surgery to spay and remove the unborn babies.