Here's a different take than you've probably heard. Everything you read, everything you hear says the WS MUST cut off all contact with OP. I totally agree. But, you can't force them to. You can make then do nothing. I came to the conclusion that what my wife does is none of my business. And I told her so in C. C got it, instantly. W protested that what she does IS my business. But, she still sees OM, she does things she knows I don't like. So, I have resolved that I have the right to tell her what my preferences are as to her behavior. I also acknowledge that she has a right to make whatever choices she wants to, and that is none of my business. It follows, of course, that what I choose to do, as a response to what she does, or for whatever reason is also none of her business. I choose to respect her stated preferences, and I choose to respect her right to make her own choices...
I have stated this all to her in counceling. The result, I believe because she saw that I truly respect her right to do what she chooses, has been that she has become much more open, much more honest.
The sad part in you situation is that she feels she must continue to lie. That means she doesn't trust that you will handle the truth. That means that she continues to hide things, to lie. What can you do to allow her to trust you enough that she won't have to lie?
All of this, I believe is the epitome of letting go and stopping trying to control the other, and working on yourself. When you stop reacting to the behavior of another, when you stop gauging your feeling and emotions by how another treats you and your preferences, you reclaim your own self-esteem. When you build your own self-esteem to the point where nothing she does can tear it down, it show more brilliantly than any nagging ever will, and make you appear tremendously attractive and self determined. You then no longer allow her to have the power to make you feel... You feel, and reclaim the power of your feelings. Then you control you, and you have no need to control her. Then it gets interesting....