I think that it's really sweet that you care and want so much to protect her. I think the best way to deal with this is on the scale of what your parents call her consistently over time. So don't act on a moment that has already passed, I think you should look instead for moments in the future. If they call her a "he/him" in the future, I would recommend a quick conversational correction and then move on, and do that as often as it takes. It isn't always easy for people to be aware of what pronouns they're saying, but that's no excuse to not try. Just a simple, "Mom/Dad." or "You mean her." just thrown in there will get the point across if they respect her. And given time I believe it will become obvious what their intention is. I really hope that they will be accepting, and that things go well for you in general.