Nightsister is a faction, not a species. There are human and non-human nightsisters. However, Ahsoka was a Jedi Padawan and then later a rebel, not a nightsister.
She had a night sister synergy as ventress is a night sister and in the clone wars cartoon she had to work with her for a while to try and clear her name in the case of who bombed the Jedi temple (And mostly because from the game angle NS are so bad they needed some help. Also from memory there was an event a while ago where you needed female characters and there weren't that many in game so they synergised her with them)
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It also doesn't make any sense for her to have nightsister synergy. Totally dumb.