I think it's a cool system. Perhaps instead of being immune entirely, Orlith just has a massive penalty to her fertlity? She'd be considered a less fecund race with like a -9 on her fertility rolls, so she'd literally have to roll a nat 20 in order to be fertile, and even then it'd be the hardest conception check possible for Isabella to succeed, and then another -5 to her acceptance check. That way it's not actually impossible to impregnate her, but it'd be hard enough that Isabella would still have incentive to find some additional means to pump up Orlith's fertility.