You're learning to sing every song that you hear frequently, and I can just picture you serenading your sweet baby sister while rocking her in her bassinet. (supervised of course). :)
You care for others, when I say ouch you always ask "Are you O.K. mama?".
By being your two worthy self your learning all the things I'm sure you'll tell her not to do; no doubt keeping her from the same fate you once endured of daily time outs. (*wink and smile)
Yes you will be the older wiser big brother. The one who picks flowers for her because you know the happiness they can create. The one who tells her it will all be O.K. with a kiss. You will be her playmate, her protector, her role model and entertainer.
You corrected a passing stranger in the store the other day who commented that you were not a baby but a big boy; saying "I'm mommy's baby." What an answer! Yes it's true, and that you will always be. My first born baby will soon take on a new role, and I think you will wear it well.