I can only give you a few things that were priceless for me. Even though the circumstances were not the same a few things my elders shared with me helped me keep my sanity.
Rick (step father) and I were married when my three kids were under 7. I did not learn the truth till my youngest was half way through his senior year. Rick never came into the truth but I feel he was favorable to it just had addiction problems. I studied 6 months before becoming baptized. The day I was scheduled to meet with my elder to go over questions was a day that I hold dear. I did not at the time but definitely do now. That morning I was doing laundry and when I removed. My sons jeans from the washer I found a unopened condom in the washer. I assumed it was his because if it was not my husband had a whole lot of explaining to do. So like every mom I panicked and did not stop to think how I might could handle it in a way that would not traumatize neither of us for life. So I called him on his cell (son #2 I'll call him to protect the guilty), at school after I had gone to work. When he answered I did not even say hi. I cold opened with a angry voice that said 'Do you want to tell me why I found a condom in your laundry?' He paused, sounded quite upset and blurted out 'Are you on your speaker?' then he hung up. I looked around the nurses desk and realized I had hit the speaker button and my voice traveled all over 5 East orthopedic ward and nurses had come out of rooms looking down hallways and doctors were at the desk smiling. I was horrified. So my plan not to traumatize either of us was a failure. Then I went to brake room to make a call to my husband. He said 'Leslie I am the one who told him to use protection.' I was numb. When we both got home that night I yelled at my husband for trying to raise a boy that was not his and was something I would not have suggested. He said he had told #2 son to stay at a friend's house that night and give his mom time to process what had transpired. I went to my elders house that night to review the questions and told him I was concerned I could not be baptized because my 17 yr old son was having sex. He asked me to slow down and explain from the start what occured. He was so KIND, SMART AND PATIENT! HE SAID that my husband having taken on the role as head of house dealt with the situation as best as he knew how. He even went on to say that this relieved me of a great responsibility. I should teach my kids about Jehovah as it was my responsibility as their mother. BUT if there ever was a good time to have a worldly husband THIS WAS THE TIME! HE said that as parents we want to teach Jehovah's standards but weigh all the information. My son was almost grown. He had never had a religious discussion in his life with his mother or ANYONE ELSE. Rick was taking on a tough roll as raising and protecting another mans children. He was thinking of STD, unwanted pregnancy, youth, and doing his best with advising on that tender subject. My elder had no children. He was around late 30's to early 40's. Use to be a writer for The Love Boat and Love American Style. He had been WORLDLY. So, after assuring me I could still get baptized I said 'Are you sure?' He thought for a minute then said the only problem he could forsee is if I was trying to hold out for the position of an elder. 😂 He told me just a few months and son #2 would be out of the house. It would be naive to think that he would stop having sex just because his mom said to. He encouraged me to not make Jehovah or meeting attendance a condition of my relationship with him or any of my children. So what I feel is If the only time you bring up spiritual things is to correct or redirect them, they'll start avoiding you along with the meetings.
With the 19-year-old especially: he's an adult now, living with a non-Witness father post-divorce. Any push from you will likely read as "the man who married Mom telling me what to do." Your best move with him is probably patience and genuine relationship-building with zero religious agenda attached to it, for a long while. Trust gets built before doctrine does.
With the 15-year-old she's still under your wife's roof and authority, so your role is more about backing your wife up quietly — not stepping into disciplinary or spiritual-lead territory yourself.
I want you to know that my husband's relationship with all three of my kids was solid. I feel like it was better than mine. I was 42 when I learned the truth. Still so eager to learn more. I'll keep your family in my prayers as well as all step families. Now I know why marriage is suppose to be to one person commitment . It avoided so much hurt in the long run. Do I wish I had stayed married to their father. No. I'm still sorting out the why.