I can't tell you what to do Jo, but if it was me - yes, I'd let them know. I think you need to try and talk to your mum again about it, and give her a valid reason for wanting to say something. You see, it doesn't matter what's happened in the past. The point is, that if your stepdad passes away without seeing the children, or they don't know what's happening, they're going to wonder why they hadn't been told, and it'll probably leave them feeling devastated for the rest of their lives. If you told them, it'd be up to them if they wanted to see their dad, and vice versa. In fact - what's your stepdad think to the idea? It might be best to ask him what HE wants - x.