Though it's very unfair on Nicola, I think it's psychologically very plausible, especially on Ginty's part. People often do resent/become angry at someone who's just saved them from doing something immensely stupid: sort of the inverse of the reflex that turns worry about someone who's just survived a close shave into anger at them?

Ginty, despite her Rupert/Crispian fantasies and flirtation with Patrick, notably failed to save him from blowing his brains out. Whereas the sister whom he'd recently unceremoniously dumped in her favour did. Also probably awakens memories of Nicola being rather more together than she was during the events of TMATT.