I was recently asked about what I thought about siblings at birth and this was my reply:
My own personal experience with kids has been that it really depends very much on the child and the environment and education around them. My own two were very informed about the birth process (some would say TOO informed :-)) and are somewhat older, so they easily were part of the process.
When my eldest son was at my 2nd birth he was much younger and still to this day feels very honoured to have been one of the first people to see his sister and be a part of the huge event.
However, some children find it hard to not be the centre of attention (depending on the age) and may distract you from your work at hand. Exposing and educating them to what the process may be like just through open casual conversation and movies helps a lot and not building it up too much.
I also think that it is important to have a person especially there for the child that they become comfortable with and you trust to react to situations appropriately. I have been a support person for quite a few children and often this has just required me taking them out to the park or some other form of distraction if things become too intense and then perhaps binging them back for the birth itself. I feel it is quite important that they feel involved in what is happening, but this does not necessarily mean they have to be there all the time, so long as they are part of the process.