Keep in mind that I haven't talked to your sister in, what, like five years? Something like that? I definitely don't know her well. That said, I remember her being somewhat. . .quirky, I guess is the word. So if it were me, I'd be trying to think of games that are off the beaten path, to some extent. For example, what about a game like World of Goo, or Machinarium? Or a combo of both? They're cheap enough that you can get both, and she might like at least one of 'em. Or what about the new Monkey Island game? All five episodes are out now, and you can get the combo back for like 35 bucks. That definitely fits the "quirky" criteria, and your sis has a pretty good sense of humor. Or at least she did, five years ago. I'm working on old data, after all.

Anyway, recap:

World of Goo
Machinarium
Tales of Monkey Island

Best I could do.