Her nephew is her niece
Sigh.
No, the argument cannot “then develop to” whatever scenario you dream up. Knowing that men are not women does not “develop to” or lead to or prompt or motivate or allow random extreme remarks along the lines of “you’re out because you have blue eyes.” Saying, pointing out, underlining that men are not and cannot be women is not random the way “because blue eyes” is. What sex people are is not random and it’s not always or automatically irrelevant. It’s not random and arbitrary to say that men should not be in women’s sports or in jobs reserved for women.
Trans nephews are irrelevant to this fact.
She wasn’t following that case? Why not? Wouldn’t following this case be an important aspect of a union leader’s job, to stay current with rulings of this kind? And the “real issues” in that case were someone lying about a fundamental, immutable aspect of his being, i.e. his sex, demanding that women acquiesce to, support, and repeat that lie, and willingly, enthusiastically place themselves in harm’s way, in his way, in service to that lie, and for his sexual gratification. I’d say that’s pretty “unreasonable.” How’s about you go “unpick” that, okay?
Chock this up to a good example of the slippery slope fallacy. If I were still teaching, I could use it as an example. It’s classic.