I've heard the same argument from 'whataboutist' rightoids from Reddit when they started bitching about Lia Thomas and people blaming Katey Ledecky, a cis woman, for being trans. Also, this same group loves to bitch about Fallon Fox forget about Patricio Manuel, complain about Nyla Rose and twist the story about Mack Beggs.
If your niece loses then maybe you should teach her to lose gratefully rather than just jumping to blame someone else for her shortcomings no matter their gender, height, weight, natural talents, hormone levels, zodiac sign, etcetera