I wouldn't be surprised if anti-gender-bias training (which, to be clear, is a good thing to do IMO) does some cognitive damage to these particular formulations

I wouldn't be surprised if anti-gender-bias training (which, to be clear, is a good thing to do IMO) does some cognitive damage to these particular formulations because questions about "sister" and "brother" get associated with training data where the correct answer was to refuse to generalize. Pure guess though.