If your narrative suggests that all women are weak, then you're being sexist. If one of your characters suggest that all women are weak, then that character is being sexist. If neither of the above applies, then nobody's being sexist. It's very simple math!
The weakness is a result of the enslavement. Bear in mind, in most, if not all, patriarchal societies, the enslavement of women, either physical or emotional/psychological is what held the distaff of the species in check, as it were. But throughout the ages, every now and then, a woman realized, "this is not right", and rose above the brainwashing to chip away at the wall of oppression. If not for the strongest -emotionally, psychologically, or physically - among any group of oppressed people those people would remain oppressed. And, sometimes, it may be a slow realization that moves that one person to act out against their oppression. If not, the American South would still be full of cotton and peanut plantations worked by black slaves. Women and blacks would still not have the right to vote. South Africa would still be the land of apartheid. Women would not be allowed to be doctors. All secretaries would be men. Need I go on? Sometimes, it just takes one to light the way for the rest to follow. But it still takes that one. So your girl enslaved is shown as weak? That just gives her room to grow, doesn't it?
I say just write it, I don't see anything sexist with it. Also every novel has themes...sexism could be one of yours if you wanted?
If the reader can understand the alien culture, they'll understand why the female is insecure and, as you said, weak. But it's neccessary for the reader to have knowledge of the alien culture, and not simply think that you enjoy writing about weak women.
It won't matter as long as the character is believable. Personally,I would be inclined to allow her to show some spark that signals strength. It's rare in real life to find someone who is either completely confident or completely broken. In a novel that's not something you want to do. You end up with 2 dimensional characters. Create conflict within the character. In this case she wants to think for herself but finds it difficult to overcome her conditioning. I think you're on to something here.
Don't know... playing devil's advocate, why does everything have to 'balance out' all the time? Are we so afraid of just expressing something without all this overthinking? It seems men worry about this more in their writing (as far as feeling they may be 'accused of sexism'), and they really shouldn't be. Of course, it would be nice if the slave gets some backbone and stands up for herself at some point. But that's got nothing to do with gender imo.
