Tofbrks, I'm so sorry your D found that info and sorry that an A has been confirmed. That's an awful comment from your W about killing you - what a dreadful thing to have written - and I'm so sorry you and your D read that.
What seems clear is that she's deep within the grip of this A and not thinking straight to write ridiculous stuff like that.
What do you do next? Well, I think you have a pretty volatile situation with an (understandably) angry and upset daughter. My guess would be that if you don't confront your W with this info in the near future, your D will.
I wouldn't involve your in laws in any way at all. If you do anything, I would link directly with your W and her only. One option might be to copy the messages that have been read and send them to her, letting her know that unfortunately your D has read these too.
What are the implications here from your end? From your POV, what is going to happen now that you have this information. What steps will you take to protect yourself (emotionally and financially) whilst your W is active in an A. These are the things to decide on and calmly communicate to her I would say.
I would encourage you to let the dust settle before you make any huge decisions (ie: to file for D. But I'd be tempted to interrupt that week at the beach with the information you have - but that's JMHO of course.
I'm sorry for how things have unfolded. Try to be the sane and calm one here and know that things will ultimately improve. It does take time, but every step forward is a step closer to a better time in your life.
Take care & I hope something here helps you
T 13 M 7 Me 48 H 46 SS 15 BD 7.14 PA D final 5.16 (H filed)
We receive & we lose, and must try to achieve gratitude & embrace with whole hearts whatever of life that remains after the losses - Dubus
