I'd start by reading "Love and Logic" & teaching "no." There is also a great book called "Siblings Without Rivalry." But keep in mind...you just never know. My 3 year old decided for almost the first year of my second daughter's life that she was daddy's girl & Sam was mine. It was very difficult. She, too, was the first, only & grandest. Now, at 4 & 7, they are great together 80% of the time. They play like best friends. But for that first year or so, while I was nursing & taking such care of the baby, it was really shakey! My oldest was in a mother's day out 3 days/ week from 9-2, to give us all a break. Eventually, it all worked out. As with most things it just took time & patience.
D.