Italian I didn't mean to imply that YOU thought of it as a risk, just that some people do.
And I totally agree that when you add a birth child into the equation, you get the risks associated with what it would mean to them too.
I think that in fostering in general, there is more risk of mistakes being made that would lead to birth parents finding out your address, than in adoption in general. Just because everything is still so much closer to the birth parents. For instance you bring the baby to a hospital appointment, the hospital sends the results to birth parents, but don't blank your address. It shouldn't happen but it does. Whereas in 'normal' adoption, the link between the child and their birth parents usually is much weaker, by the time you take your adoptive child to a hospital appointment, there is little chance they even have the birth parents' address on file, so even if they meant to, they couldn't make this mistake.
Because we are still considering CP (we are by no means sure that it is for us), we are absolutely NOT talking to DS about having a new sibling. I think it would be very hard for a child to have a new baby introduced as their sibling, then to be moved away after maybe a year ..
We think that IF we do CP we would communicate (to everyone, not just DS) that we were fostering. Which is nothing but the truth! And if baby were to stay, we would go on to say we were now going to adopt our foster child. I think gaining a foster sibling as a 'forever sibling' would be much more feasible, than potentially losing what was thought of as forever sibling.
So as long as we are still considering CP, we are not mentioning possible future new siblings to DS. Instead, we have gently introduced the topic that some babies can't be looked after by their parents, so someone else needs to look after them, and maybe that is something we might do.