Is it permissible for the questioner's paternal half-sisters to uncover themselves before her suckling nephews, or is this limited to her full sisters only?

Lactation establishes the same prohibitions as lineage, as the Prophet, peace and blessings be upon him, said: "What is prohibited by lineage is prohibited by lactation." So, whoever suckles from a woman becomes a child to her and to her husband, the owner of the milk. If your paternal half-sisters were nursed by their father's wife, they become foster-sisters to her children, and the children of this woman become mahram (unmarriageable kin) to your paternal half-sisters, because they are the children of their foster-sister. This is known as "the milk of the male." It is a condition for lactation to establish prohibition that there be five full, satisfying sucklings within the first two years of the child's life.