A.,
Congratulations! We were blessed with a little girl and my husband fully expected we would be blessed with soley girls, I think he almost fainted when we found out that a little boy would join our family! :-)
Mine are almost 3 years apart, now 5 and 2. They have a fabulous relationship, obviously they are not in their teens and time is still to come, but I truly don't think their dynamic will change much. Don't get me wrong, they have disagreements, but not many. One of the greatest pieces of advice I got when I became pregnant with my little guy...Don't make her a Mommy. Another words, don't ask her to fetch every little thing for him or for you for him. The approach I took was if she directly asked to pick out his clothes, I allowed her too. But I never asked her to get anything, even as simple as a diaper for him. I had complete buy in to this advice as I am the oldest and what I was told made since...If you make her feel like a Mommy now, she will always consider herself in charge of what he is doing, etc. UGH, that hit home! ;-)
I wish you well and trust yourself, allow your kids to be who they are and they will do great! Oh, maybe check into the book Siblings Without Rivalery. Awesome book! Hard to implement concepts with an infant, but great knowledge!
Wishing you the best, J.