Are the daughters of the questioner considered sisters to the children of her sister by rada'ah (nursing), and thus is marriage between them forbidden, given that the nursing was done with a pump and did not exceed five times, and both girls were infants in the incubator?

There is no difference in breastfeeding between drinking directly from the breast or drinking milk using a bottle. What is considered legally binding are five satisfying breastfeeds. So, if your daughter and her sister drank from your sister's milk five satisfying breastfeeds, they become your sister's daughters by suckling and sisters to her children, and marriage between them is prohibited. However, if the breastfeeding was less than that, it is not considered legally binding.