I'm not sure, some people let them linger about and watch, I think this could be really healthy for them, seeing something as normal and natural as birth happening, along with seeing their sibling being born. However, if they're the tearaway type, they could just add to your stress levels and not be nice to have around at all. Especially as birth can be long and boring. I'm also slightly worried about the birth turning out to be a bit traumatic (like my last one) and obviously I wouldn't want my little one to see mummy in distress etc. So, although I love the idea of having my quiet little angel seeing his little sister come into the world, I might get my mother to take him for the night instead.
On the other hand I think it could be very healthy for him to be there, he'd understand exactly where the baby had come from and wouldn't come home to find a new small person had moved in while he was gone, just waiting to snatch his mummy away. I think his being there and being involved in some way would be much healthier... So really? I'm not sure. That's yet to be decided. He's only 18 months, if he was older, say 3 or 4, I think I'd definitely prefer him to be there.