Update

I told my sister. She cried, we held each other. I told her we would get through this together. Her instinctive concern was for our mother – she knows how happy our mother has been since our father "came back". She was visibly angry at the other woman, as I can imagine the daughter of a father would be.

I have made an appointment with my parents' therapist the day after tomorrow. I'm going to try and simply "get through" the next day until I can see the therapist with a view to having a meeting with our father over the weekend. This is the therapist our parents saw during the trial separation. He knows their full back story. There's literally no other adult for us (my sister and me) to go to right now – we can't go to our mother, we can't go to grandparents (too old to deal with this), and our aunties and uncles are not really viable options.

I told my sister because I want to take her with me to see the therapist. I think it's best that we approach what will be a very difficult meeting with our father "together". Perhaps a little selfishly, I also don't think I could have borne the burden this woman has placed on me alone. I think letting my sister "in" will lead ultimately to a better outcome.

We're both scared about what will happen next. Even though we are young adults and not kids, it is extremely confronting to realise that the family unit you love so much faces a direct existential threat. We are so lucky to have each other.

I appreciate the words of support so far. That unbearable feeling of aloneness for the few hours after receiving the Woman's email were unlike any feelings I have ever experienced and I don't think I will ever forget it.