It is probably not my place to say anything but I can feel a certain resemblemence to myself. Not being used to BDSM and only just starting off with my husband SIR I can only say that I'm very lucky to be introducing BDSM into your lives.
But for someone who has an overwelming urge of need to be a sub, you 'may' have given her the opportunity to feel wanted. Not blaming anyone as sometimes emotions can make decisions rather than actual thought. It will take time but I agree that you have to take a stand now rather than later.

Is there anyway that you could have 2 subs. It may be more work involved but this could also be an advantage for all three. They could both prove their loyalty to you as afterall it is an online thing but are you looking further on than that?

It is only a thought so please don't take this the wrong way. I just know how strong the feeling is to be a sub.