Christine Something worse. She told them as an achievement
Now that's interesting. Sure, I am rather naive here -- I don't know those relatives and their motivations -- but if your mother actually praised your belting as an achievement when talking to them, this sounds to me as a pretty logical starting point for the "Good for one single, good for all single"-argumentation. (Which should help her understand, what she actually demands, as a next step.)
I see that her seemingly higher credibility might be a complication at first. I understand her counter-argument against the belt is "I'd never indulge in sexual activities so I don't need a belt." But then again, everyone can and probably will use that argument at some time. Towards oneself or others. Apparently she believes to be sort of "above" carnal desires. Yet, given the way how the primal parts of our brains work, I'd argue that she is fooling herself and those relatives.
Throughout history, people claimed to be unaffected by desire, ... And despite all those words lost lives, their dignity, kingdoms, just everything, because of those nasty neurotransmitters, neuromodulators, neuronally effective hormones, ... It's biology. And people's ignorance, overbearance, their hubris, before the fall ...
You could start get into contact with those other relatives. The people to whom she extolled the achievement. Maybe you have to go on a 1:1 basis and take some time to lobby, persistently.
Jumping in with a snotty "I should get rid of the belt!" might leave you in a bad position, no doubt. Yet arguing "She [ Christine's mother] is as single and susceptible as I [Christine] -- despite what she tells. I'd accept easier if we both wear a belt for a certain 'trial' period controlled by reliable relatives" could give you some leverage IMHO.
I agree with the others above: Even if you eventually still want to get rid of the belt, your chances of succeeding are much higher IMO, if your mother has to taste her "achievements" herself, before. I'm pretty sure, she'd change her perspective in some way after experiencing belting on her own -- and it can't really become worse as it is now, could it?