I haven't read the entire thread, but just want to emphasize this:
I don't think it's a great idea to try and not reveal the A because both of them are old enough to have accidentally witnessed something about it already.
My DDs (step, but they're my daughters to me), were 17 and 19 on DDay. They had already witnessed a LOT that was confusing to them. They were also, unfortunately, witness to DDay, since XH decided it was a good idea to bring another woman into our bed while they were in the house.
However, even if it hadn't blown up in that specific way that they had to see, I do firmly believe that telling them the truth was the right thing to do.
They had SO many questions. About his incredibly confusing behavior. About his lies. There had been a really strange feeling around the house for months that nobody could quite put their finger on, but the girls and I were all in a perpetual state of vigilance and confusion (the effects of gaslighting as we now know). And once the A was blown open in front of their face, it was like they finally felt comfortable to let their guard down and ask all of the questions they had been bottling up inside. It was truly like the flood gates had opened, and I was made aware of far, far worse issues than the A based on what they divulged once they finally felt comfortable to do so.
It is more than likely that your WGF has said or done things in order to "cover" for the A, that have left your kids feeling uneasy or questioning whether their mother is lying to them.
It was incredibly difficult to hear these things, to see how deeply they were affected by all of it. They both had numerous breakdowns shaking in tears as they pieced together the lies he had been perpetuating. I had to keep a straight face and comfort them when everything inside of me wanted to go strangle him on their behalf.
But I cannot imagine how difficult it had been for them to keep all of that in. That is a trauma they absolutely didn't deserve. And while I'm not happy DDay happened in the way that it did, I am happy that the end result gave my daughters the freedom to speak their truth and to ask any and all questions that helped them make sense of their lives. And for the opportunity to reassure them that none of the horrific behavior he had directed at them was their fault.
BW
DDay Nov 2018
Many previous DDays due to his sex addiction
Hurt me with the truth, but don't comfort me with a lie.
Love is never wasted, for its value does not rest upon reciprocity.