I have absolutely LOVED sharingthe experience of meeting our new baby with our daughter. It is so different to when it was just her.
From day one I thought it was magic. She was gentle, her eyes were wide with wonder. She thought this new baby was the best thing ever.
And as he's grown she has been his favuorite thing in the world. He just watches her, all the time, and laughs when she bangs him over the head (which makes discipline very difficult)
We were pretty open minded about the effect it might have on her, bad behaviour, regression etc. but none of those things were true. It took us a bit longer to be ready for things like getting rid of her dummies and night toilet training (we're still not there) because we needed to wait for a time that we were ready to put up with less sleep, more laundry etc, but she's been pretty cruisey.
Some things are a bit harder to come by...sleep, quiet time, but some things are much more abundant...precious moments, love.
Good luck
x