When our son was about 18 months old, we told him he'd be a big brother soon. When he was 20 months old his baby sister arrived. But before then, my son and I hung out all the time. We went shopping, played at parks, took long walks, did a stint in a baby and me exercise class, stacked blocks, read books again and again. And again.
When I was still in the hospital with our new baby girl, he would be really watchful. Not wanting to hold her, but he wasn't wild about other people holding her either. He got a little protective and curious. My wish for them at the time, and actually still is, was that they would always love each other. They might not always like each other growing up, but they would love each other and be there for each other, no matter what.
Here's a little glimpse of their relationship when she was about 3 and he was almost 5. They're in the backseat and I'm about to go through the car wash. My son had been before and he thought it was a hoot. So, let's try it, right? She gets nervous, says she doesn't want to do it. He says, "It's ok, hold my hand." (awwww) and he reaches across the middle back seat and holds her hand. They talk and giggle through the car wash. The moment we drive out, she smacks his hand. WHAT? I think she had built up some anxiety and couldn't express it. She had no reason, but by the time we were home they were giggling again.
So, I think it matters less how they are introduced in the beginning, and more what you try to foster in those early years. They don't have to always like each other, but they have to love each other.