Is it sexist if the girls like her too?
I never thought of BOF3 Nina as a particularly sexistically (!?) written character, though I never had her in party (BOF2 Nina, on the other hand, rowrr! -- uh, even though I've never played BOF2, just seen the artwork and sprites). I always had Momo in when I felt the party needed some estrogen content because Momo had a bazooka, which beats any other weapon.
Whether something is still sexist if the sex typically targeted by the supposed sexism (say it three times fast!) is a fantastic question. There seems to be a consensus among most feminists that a pro-life stance is automatically sexist even if women espouse pro-life views; I'd be willing to bet that pro-life women have an even harder time trying to self-identify as legitimate feminists than straight men do. It is a common attitude that the Islamic hijab is sexist even if women find a humanist interpretation in it and embrace it (though by no means a consensus, with a sort of cultural relativism seeming to become more mainstream in Third Wave feminism). A second wave feminist might pronounce makeup marketed to women sexist, even though third wave feminists like Jessica Valenti embrace it.
In short, it probably depends on who you ask, as feminism is such an attitudinally(!?) fluid movement. I think it's certainly possible for a woman to promote things that are counterproductive to women's rights, just as a few African Americans have owned slaves in US history. I seem to remember a quote from Queen Elizabeth I to the effect that women's rights were somehow not necessary in her day and age, but now I can't find it -- was it apocryphal? I could have sworn it was in a history text I read in high school.
But as for the specific issue of judging whether a videogame character is sexistically(!?) written, I think it depends on whether the character tends to make players of either gender respect the character or dismiss the character as either a ditz or the bauble of a hero more central to the plot. Despite all my sensibilities about Tifa's appearance in Final Fantasy VII, I admire her for her inner emotional strength and perseverence more than any other single videogame character.
I like making up new words. Hopefully they're grammatically correct if not semantically correct.