I find it plausible the entire story is fabricated just to stoke division.
I find it plausible that 'relative' was indeed an exhibitionist walking around with an inadequately-contained semi (utterly inappropriate, in case of any confusion).
I find it plausible that the 'relative' was acting out of naivete rather than bad intent, may or may not have been fully covered, and was the victim of a malicious report from that mother.
It all sounds very embarrassing all round.
I imagine that for most trans people, the near-nudity of swimming is really difficult to navigate. My local leisure centre runs a specific trans/NB session and i think that's great (just as I think specific men's and women's sessions are great).
I am however drawn to the prospect that this NB / female-identified person was clearly read as presenting male in their non-swimming clothes - which does feel rather different than eg a mostly-passing trans woman being spotted as male-bodied in a swimming costume.