There's a very important difference in amp requirements here.

Modern keyboards have circuits that generate all of the tones and effects internally, so they sound best through an amp and speakers that reproduce that sound accurately. Guitar sound is the combined result of contributions from the guitar, effects pedals, the amp, and the speaker(s). Not the same thing at all. The cruchy distortion that guitar players love would trash the sounds produced by a synthesizer.

Jon Lord of Deep Purple used to play his Hammond B3 organ through Marshall 100 Watt stacks. But that sound was more akin to the way a guitar player uses an amp's distortion to define his tone. I have loaned my old Ampeg VT-22 to a keyboard player, and it worked well for him only because that amp has a lot of clean headroom, and he didn't push it hard. In fact, I have used this amp for vocals a couple of times. Which brings us back to rockonn91's point - keyboards need clean amps, like a PA system. Not suitable for guitar, unless you only use a clean amp setting, and base any distortion on pedals.

Sharing is comendable - good on ya! Having said that, it's your amp, and you play guitar. So get a guitar amp that gives you the sound you want, and continue to share it with your sister if you wish. If she likes the sound she gets from it, good (Jon Lord would approve), but if she wants something more suitable for her keyboard, that's her responsibility.