Want to contact half siblings BUT

So, I'm 33 now. I found & had one meeting with my birth mother 2 years ago... I could write a book, but basically she didn't want to stay in contact and didn't want to tell her other children about me. She got married years after she gave me up and the husband knows but she never told their kids. They are all adults now and I very badly want to contact them. However when I had that one meeting with her I very impetuously told her I wouldn't "go behind her back" to tell them. I didn't fully realise at the time how important would be to me and I now bitterly regret it. I wrote her a email some months ago and told her so and asked her to tell them finally. I never received a response. I don't want to break this promise even with how unconsidered and regrettable it was. I also truly believe they have a right to know and make this discision for themselves, even if their answers are all no. Does anyone have some advice??