Introducing my children to their half-sister?

Situation is super weird. Sorry + thanks in advance.My ex-wife and I had 4 children together (Jo 20F, Ripley 19M, Cosima 19F, and Morgan 17M) before she left us 10 years ago. The day she left, she was supposed to pick up the kids from school, but decided to pack a suitcase, leave a note, and just vanish. I was unreachable at the time, the school couldn't get in touch with the emergency contact, and they were left there for a few hours before I had to come and tell them that their mother had left. As a result, my children are deeply traumatized.This past January, she told me she was moving back to our province and actually apologized for leaving, saying she had been overwhelmed at the time. She wanted to reconnect with the kids and attempt to coparent with me. I have not forgiven her, our kids have not forgiven her, and it is extremely unlikely she will ever be their parent again. However, because my children have expressed interest in reconnecting and gaining closure, we have been working with professionals to facilitate their relationship.Even though my children are all adults (save for Morgan), they asked me to act as an intermediary in this situation and help them navigate. We have drafted rules for this situation as a family - she is not allowed to take them to her home, she has to have all visits be supervised, she needs to give 24 hr notice, she cannot bring a guest with her, etc. etc. - and gave copies to myself, the kids, my husband, and my ex-wife. The kids get final say in everything.This weekend, my ex came to the house while my children and husband were out. She had a little girl with her, who she explained was her 9 year old daughter, and told me she felt bad for not telling our kids about their half-sister. I was upset, so I asked her to leave. I let her know that I did want to introduce them, but the kids couldn't be blindsided by this and I needed to discuss with our family therapist, my husband, and do some research. She would not leave, and then she finally took her child and left after I shouted at her - which I should never have done.After talking to my husband, we decided that we need to tell our kids ASAP. We have made our calls and got some great advice from our therapist, but we wanted to reach out to fellow parents and see if anybody had some additional tips/advice. We want to explicitly broadcast the message that just because their mom has stuck around for this kid doesn't mean that they were... To blame, I guess? Our kids have dealt seriously with a feeling of guilt and responsibility.​... Any advice? via /r/Parenting https://ift.tt/33Lm7Rj