Is the suckling girl considered a sister to the neighbor's son whom her mother breastfed, and are his full brothers and sisters also considered brothers and sisters to the suckling girl?

If the suckling is valid (five known sucklings within two years), then it spreads the prohibition between the suckled child and his wet nurse, and her ascendants and descendants.

So, your sister who was suckled by your neighbor, the neighbor becomes her mother, and her sons and daughters become her siblings.

And the neighbor's son who was suckled by your mother becomes a son to your mother and a brother to you and your siblings, while his brothers and sisters do not become your siblings, because the determining factor is suckling from the breast.