Are the paternal female cousins considered maternal aunts to the children of the first wife's daughters? And is it permissible for them to unveil themselves before them, given that they all nursed from the second wife?

If the paternal uncle becomes a foster father to you (fem. pl.), then all his sons and daughters are your foster siblings. Accordingly, you (fem. pl.) are maternal aunts to the children of your paternal uncle's wife, from whom you (fem. pl.) did not nurse, and it is permissible for you (fem. pl.) to appear before them without hijab.