Here's my thinking on this. She likes the "family" but doesn't love it. She may "like" you, but doesn't love you.
My guess is she feels extremely guilty for feeling the way she does. She may be being told how great she has it and she can't understand why she's not happy. This could be WHY she says she doesn't know....because she doesn't. Honestly, I think SHE is struggling to come to terms with her feelings of not being as invested in the family and you as she should be. She keeps trying to feel differently, but just can't. Your post about her saying it's weird being jus the 2 of you....that threw up flags for me. IF she isn't cheating physically, she is likely cheating emotionally. She may have found someone that brings back all the spark and excitement of a new relationship....and this fuels her feelings of guilt.
With that said, I'd tell her it's done. That she needs to move out (she asked if you wanted her to, she is willing) Let her have a little freedom and see if that opens her eyes to what she wants. My guess is someone is making her feel the fun and excitement and she is thinking "grass is greener" It never is, just different grass. She won't know until she finds out....however, YOU have given everything you can from the sound of it. Time to get yourself right and be there for your kids. No need to villianize her....she just doesn't feel the same way.