S15 will tell his sisters what is going on in the family. Unlike adults, kids know there should be no secrets within the family. And your wife will likely seize the opportunity to accuse you of informing them wrongly and undermining her good efforts (hah!).
Personally, I told my kids right away, "Dad is leaving us." Yup. No sugar coating. And the inevitable happened. The kids all took sides. Or side. My side. Family against non-family. Did it make a difference? You bet it did!
So she will accuse you of setting the kids against her. Don't feel guilty. She had her chance. You were going to go along with her method and her timing. Until she blew it off. Not your fault. You don't believe in keeping secrets in a family. Not secrets that are anti-family. You are not her partner in this. You don't have to go along with her. But finish the job you started. Tell the girls. They already know.
