If there wasn't anyone in the women's communal area at 8.50 then it's doubtful there would have been many in the men's, either.
How about when they came out of the pool and needed to changed again, was it empty then too? Or were there women in there who didn't want a near-teenager in there with them?
What would your sister have done if women/girls walked in while your DS was changing? Made him sit in the cubicle for ten minutes until they'd finished?
Presumably there were toilet cubicles in the leisure centre?
Why couldn't they have changed in there?
You and your sister are ridiculously overprotective. Do you think many peodophiles are prowling local leisure centres at 8.50am on the off chance a child comes into the changing room alone?
If he's not old enough to get changed himself at 12, when will he be? I would understand feeling cautious if he were 9, but he's half as old as the maximum age again, ffs.
The height of arrogance to see a sign and instead of thinking of an alternative (going home, changing in the car, changing in a toilet cubicle, putting trunks on over underpants and then pulling them out from underneath), just deciding 'Oh that applies to everyone else, not me and my precious coddled nephew/son.'