So, my laptop hard drive got a bug and now will not work. As of now, I'm using my sister's Macbook, and I want to play on my Terraria character/world. Is it possible to do this without the use of my original laptop?
If you have your characters and worlds linked to Steam Cloud (the cloud button on the character/world select screen), they will automatically be downloaded to your other PC when you start playing there. You will however need to do this beforehand.
If this is not the case (because you either didn't do it, or bought the game from GOG), you are better off trying to get your laptop fixed or rescuing the characters and worlds from it.