DD was 21 months when DS was born. She would point to my belly if I asked her where "baby brother" was but I don't think she really understood before he was out. When we came home from the hospital DD was there (she was being looked after by my parents in our home), she was so happy to see us and didn't even notice the bucket seat. I told her "Baby brother isn't in mommy's belly anymore, he's here now! Do you want to see him?" She was very curious and kept pointing at him and saying "baby!" but after about five minutes she went about her regular routine as if nothing had happened. His being born was really a non-event for her, for the first six months of his life she barely paid him any attention. I mean she would go give him a kiss if I asked her to, or if I asked her where DS was she would point to him, but she didn't really seem to care that he was there, even when I was nursing or cuddling him for hours on end. I was surprised since she was very attached to me before and I thought for sure she'd be jealous! It wasn't until he started crawling and stealing 'her' toys when she started paying more attention and getting jealous... that's when all hell broke loose.
She loves him and thinks he's fun to have around nowadays but they also fight a lot!