A couple of thoughts. First of all, I'm in a similar position. My hasband had the affair. So, that's where I'm coming from. My first thought is that when we were thrown into this situation by no fault of our own, we lose control.It's like we're in a vehicle, but we're in the back seat with an unknown driver going 100 mph. There's nothing to hang onto and no way of stopping. So, we have to figure out ways to regain some of that control. It sounds to me like you have a bit of control over this situation, if you keep her secrets. And that can be a good thing.
A word of caution though; I really do believe that we are only as healthy as our secrets. Keeping my hasband's affair a secret for many years, ended up taking a huge toll on me. My mental healthy suffered. My physical health went downhill; I went from being a healthy person, to a person 50+lbs overweight (and that's a whole other story), with high blood pressure, and ulcers. My therapist helped me to realize that my marriage (and secret keeping) could cost me my life. I had uncontroled and very high BP.
The way I look at it now is that I have given up my roll as the keeper of the secrets. If I want to and feel like it, I tell people that my Christian hasband had a long term affair. Don't get me wrong...I'm not vindictive and I don't just blab it all over. But I am free from feeling like I have to keep my mouth shut. And that is so freeing! You see, what his secrets did was keep me from getting the support I needed. You can't get support if you don't tell anyone what you're going through. So, I guess what I'm saying, is be careful. If you feel like keeping the secret is too costly for you, you may have to rethink things.
Just my 2-cents. ~L