There really isn't a demo, but you could try Docking Station which is free for download at
creaturesdockingstation.com. That is the downsized version of Creatures 3, which is frankly more complex then the first two.
The Creatures series is time consuming, but in an addictive good sort of way. People can play it many ways. Some of the more common ways are focusing on rearing or breeding (the genetics aspect).
If your sister isn't good with computer games,
I would definitely recommend staying away from Creatures 2, because while it is not as complex, it requires downloading and a bit of research. The creatures in the game didn't get properly sorted out before the company released it, and you have to find third party solutions to supplement game play.
To put it simply, it's buggy.
That said, there isn't a way to demo the first game, so I'll have to describe it to you. The first game is more hands on, you have to baby the norns. In Creatures 3, and it's add-on Docking Station, the creatures take care of themselves for the most part. You can walk away from your Creatures 3 game, and not expect everyone to be dead when you come back. Creatures 3 and Docking Station, in my experience, are better for players who are more interested in the genetics, breeding, and third party add-ons.
If you're looking for a cute game that's more about raising them and taking care of the creatures, go for Creatures 1, or for GOG.com, the Creatures Albian Years package. While you may not use Creatures 2, it's the easiest and cheapest way to get the first game.
Note: I ignore the existence of Creatures Village, which is the kid version of the game, because the game frankly look terrible and is nothing like the rest of the series. It's very dumbed down, but that just might be my childhood nostalgia talking. It never really fit into the rest of the series if you ask me, it's kind of a separate entity. (sorry if I just stepped on some toes...!)