Well, first and foremost, never let a clefairy anywhere near your research equipment.
(I … what? —LH
It’s valid advice! —Bill)
In all seriousness, it’s important to know first how your sister cares for her clefairy normally and stick to those routines. Clefairy are highly intelligent, extremely curious, and slightly mischievous pokémon, and if you deviate from their routine and pamper them in the slightest, they’ll attempt to take advantage of that. Write down everything your sister tells you and stick as closely to that as possible. Most importantly, never let your guard down. Clefairy are cute, they know this, and they will take the first opportunity they can get to disappear on you if you take your eyes off them for a moment.
That having been said, remember that clefairy are heavily influenced by the moon. If your sister’s clefairy is staying with you for more than a month, be prepared to allow it outside for its monthly moon ritual (an energetic dance beneath the light of the full moon). If you have any other pokémon or soft toys, it’s important to have them ready, as clefairy often seek warm, soft things to cuddle next to after it wears itself out from its dancing. Otherwise, a soft bed—preferably one with plenty of blankets it can burrow into—will do just fine to make it feel comfortable.
Additionally, clefairy are predominantly cave pokémon, so they can be kept occupied with boxes or special “clefairy trees,” or scratching posts with multiple branches. If you don’t wish to buy a clefairy tree just for this stay (which is understandable), be sure to keep an eye on your sister’s clefairy to keep it from climbing your furniture and burrowing into your laundry piles. If you can get a few cardboard boxes your sister won’t need for packing, this should be enough to keep her clefairy in line. You likely won’t need to trim your clefairy’s nails (which is an ordeal), but definitely be warned that some clefairy will claw at furniture. Supply your sister’s with soft toys to keep it from doing this. Finally, jingling toys and shiny, crinkling objects are also excellent ways to keep a clefairy occupied, but ask your sister for her clefairy’s preferences.
On a temporary basis, pokémon in general can be fed kibble, but should you wish to give your sister’s clefairy a treat at all, give it nuts or berries. Don’t feed it fairy cakes. It’s a misconception that they like those, and in any case, you very likely don’t want the resulting sugar rush. Of course, also keep a dish of clean water available for it as well.
In terms of waste elimination, most house clefairy relieve themselves in covered litter boxes, but some have been trained to use toilets. If your sister’s uses a litter box, you should change this daily if possible. If your sister’s is toilet-trained, just be warned that toilet training does not include teaching them how to flush. Be aware that an accident may happen because clefairy, normally timid pokémon, may be alarmed by any change of scenery. Just have patience and let your sister’s know where its litter box (or the toilet) is.
As for cleaning it, clefairy can self-groom, but once a week or if it gets into anything particularly vile, you can give your clefairy a bath in a sink using pokémon soap available at most pokémarts. Some clefairy are also trained to brush their teeth on their own; your sister will likely have a toothbrush and a tube of toothpaste specially made for hers that’s part of its routine. If it isn’t, don’t worry about it; clefairy teeth can go for a week without being brushed.
A few notes on some common behaviors: clefairy can largely entertain themselves (especially if you give it boxes), but they also enjoy being petted. If your sister’s nibbles at you, that’s usually a sign that it trusts you, and if it butts its curl against you, that means it likes you enough to mark it as its own. (The curl contains scent glands, much like a cat’s cheeks.) Meanwhile, another common sign to look out for is an uncurled tail, usually in conjunction with paler fur. This is usually a sign that your sister’s clefairy is getting sick, so it’s important to take it to the pokémon center as soon as possible. Finally, if your sister’s clefairy starts vocalizing frequently at night, that’s a good sign that the full moon is approaching or that it’s already here. Let it outside and keep a close eye on it for dancing.
Finally, a warning: a clefairy that gets too comfortable in its new habitat will usually try to nest. That doesn’t mean it’s trying to mate. Rather, it means that the clefairy will assemble materials to fashion for itself a nest—materials that usually include soft objects, such as clothing. Every clefairy has the ability to generate a strong adhesive in the backs of their throats that harden into a crystalline substance when in contact with air. These clefairy then use this substance to glue together materials to form that nest—which is to say that if you start noticing your sister’s clefairy gathering your personal items into a pile, it’s a good idea to keep it from spitting on them and gluing them together. If you can’t stop it from doing this, all pokémarts sell cleaners that can neutralize this substance, but it will still be a bit of a challenge to clean. Don’t lose your temper at your sister’s clefairy (for this or anything else), however. Yelling at a clefairy is a great way to scare it, which then may drive it to hide somewhere in your home or even run away—neither of which will end well for something as small as a clefairy.
But most importantly, never let one anywhere near your electronics. Just … just trust me on that.
Best of luck, anonymous!
The Pokédex According to Bill 