We, and Harry, only suspect Nemesis can't infect him. We don't know that Elaine is star born.
There is circumstantial evidence, that either she isn't a star born as hoped, or she took a different turn, and her path is way different.
Evidence for; If she was born about the same time as Harry, the stars would have been aligned for
her also.
Justin pulled her out of the orphanage, or we think he did, we don't know her full story, for some reason presumably because he felt she could be.
Evidence against; Justin was able to enthrall her.
She fled to the Summer Court after Justin died, and Aurora began to show symptoms of infection.
In Summer Knight though in the end she did help Harry, her actions were suspect of infection when you look back.
Shrug of shoulders; In White Night she seemed very normal, very level headed wizard type lady..
Verdict? Too little information to go by..