Are all of the brother's daughters forbidden to the questioner's son if the brother's wife breastfed him once, twice, or thrice, knowing that the questioner's son was sick with pneumonia and was taken to the incubator?

The suckling that establishes prohibition and spreads the حرمة (sacred inviolability) must consist of five known, satisfying sucklings, according to the preponderant opinion. If this is not established, then no mahramiyya (prohibited degree of kinship) arises between your son and your brother's daughters. And for suckling to be established, it is sufficient for a woman whose testimony is acceptable to bear witness to it, as is the position of the Hanbalis.