What is the ruling regarding the daughters of the second wife for a child who was breastfed by the first wife?

The infant becomes a brother to all the children of the milk-giver, whether they are from the nursing mother or from another woman, because the Prophet, peace and blessings be upon him, said: "Suckling prohibits what lineage prohibits." And a brother from the father is a mahram to all his sisters from his father.