Some of the questions to ask yourself (and her):
- Why are you wearing a belt?
- Why does she want to wear a belt?
- What would your role be in her belt wear?
Like everyone above, I also do not think you should get in the way of her getting a belt, but I believe your most important role is to provide practical advice. Like how does belt wear affect your life, other than the parts it is supposed to affect of course. And what sort of belt she should get started with.
If either of your interests is purely based in kink, that should probably be the end of your involvement; it is not good to mix sexual play and family. But if you both have other reasons, I would think it would be fair game for you two to pick out a starter belt for her together and for you to encourage her to get used to the sort of wearing schedule she is interested in, whatever that may be. From only wearing it when partying to prevent any drunk accidents to 24/7 wear aside from hygiene/health/safety breaks, the possibilities are endless.