Is my grandmother's nephew, whom she breastfed (along with my paternal aunt), considered my paternal uncle, and is it forbidden for me to marry him?

If your grandmother breastfed her nephew with a prohibitory suckling, he becomes her son through suckling, and her children's siblings become his siblings through suckling. Thus, he would be your paternal uncle through suckling, and you would be forbidden to him. Prohibitory suckling is that which occurs within the two years (the first two years) before weaning. Scholars differ on the number of prohibitory sucklings. The preponderant view is that five separate sucklings are required, while others believe that both a little and a lot of suckling establishes prohibition.