Why did your sister need a $2400 apple laptop if not for the speed and memory? If you are in the US at least you shouldn't have to spend that much for a new Mac. The 13" MBP starts $1200 and you can get a 15.4" MBP starting at $1700 from Amazon, with free shipping and no sales tax (in the majority of states).
It's not about need, she bumped the RAM, Disk, and added the AppleCare Protection Plan because she felt it was a reasonable thing to do. Honestly, spending less than 3k for something you are going to use several hours a day for a few years seems reasonable to me.