Well... if you have one nice desktop and you wont need the laptop, while the laptop is your sisters-in-law only computer, i think you should let it with her.
I think two persons with one computer each is much more rightful than one person with two computers and other with no one. And the fact you wont see they ever again doesnt change this.

EDIT: And if your parents get pissed cause you did the more rightful thing to do, just tell them you were only being rightful. No parents should complain about their sons/daughters behaving righteously.