Jenna I think you should use this as an opportunity to talk to your nieces about their experiences, what you could have done better, and what you can do to make things easier moving forward.
I think lost in a lot of the conversation about your experience is their experience and their expectations. For instance, when I was a teenager I would have liked to wear a chastity belt and still would like to, but I don't think I would have been comfortable having my parents as a key holder, because that is a lot of trust to give someone.
Likewise, you have your reasons for wanting them to wear a chastity belt but it is likely that their reasoning is different and you should be cognizant and supportive of whatever those reasons are.
Wearing a chastity belt is also a huge lifestyle change and intentional or not, it makes them dependent on you. You should always double and triple check that they are still able to live the life they had before and do all the things that they find fun and exciting.
I think that you are a good role model, your strong, committed to your beliefs, and clearly very successful... But sometimes you can't always be in control, and I think that is one of the things children teach you very quickly. Make sure you are listening to your nieces and letting their voices be heard. Maybe let them decide if you should meet up with this mom.