A few tips if you're thinking of guinea pigs.
Firstly, if you want to do the best for the piggies your sister is going to need to build a CC cage for them. The cages sold in pet shops are not nearly large enough to house them, can't believe they can actually sell the bloody things. I originally got my two piggies one from a pet shop and they kept fighting until I made them a bigger cage. For two guinea pigs you need 2x4 grids (I'll mention grids in a minute) The cages are simple to make though (and cheaper!), so don't worry. I popped down to my B&Q and bought the cube mesh grids from there, they are slightly tricky to find here in the UK so
here is some more information on where to get them, they can be found online it seems. I got some cheap lino for the flooring along with the grids and that's all you need. It makes moving the piggies easy and cleaning is far easier with these than pet shop cages. All the information you need is on this website, including how to make your own!:
http://guineapigcages.com/index.htm
Another tip I recommend is reusable bedding. I stock up on cheap fleece and towels from Primark and it's great. On the bottom layer you have newspaper, then a towel and then the fleece goes on top. This bedding lasts a good week or so, so you don't have to change them so much. To clean up I just grab the fleece and towels, shake it off into a bin bag and then they go in the washing machine without fabric softener as that makes the fleece waterproof. The idea is that the piggies wee and it soaks through the fleece and towels and into the newspaper. The biggest plus to this bedding is that it barely smells, I found that all the sawdust made their first cage set up stink.
If you're thinking of getting guinea pigs then I suggest that you look into rehoming a pair. I was surprised at how many places there were with unwanted piggies. The good thing about adopting is that they've usually been neutered and treated for lice, something that is definitely not guaranteed at pet shops. I have a male and female from a shelter, they've both been neutered and they both get along great.
Bear in mind that piggies last to around 7-8 years, so they are not a responsibility to take on lightly. They are cheap to keep if they're fit and healthy, but my two both had a bad spell of getting ill at the same time. Tantor formed an abscess that went from really small on a Saturday evening to fully blown and burst the next Sunday. On top of an awful smelly bus ride covered in abscess pus I ended up paying nearly £200 in vet bills because it was a Sunday, for a relatively small clean up. Soon after he got another lump that I paid to be removed. It was definitely worth it, but just a warning that those kinds of costs can happen. He's been fine ever since though and no problems at all now!
Quite a few pet food has also said on the back that piggies only need fresh veg once a week or so. Really, they should be having something fresh everyday. Terk and Tantor here love carrots, cucumber, dill, spinach, beetroot (although be warned they look like they're bleeding from the mouth when they eat it!) and especially the lovely green carrot tops.
Something else I've noticed and always warn people about...Pets at Home have been selling those massive ferret exercise balls that say 'suitable for guinea pigs' on them. I've gone in and told them countless times, they are not suitable for piggies at all. Their spines are just not made to use them. So if you're looking for toys, do not get them one of these!
Hope this helps!