I don't know anything about degus, but I do know lots about guinea pigs. I think she would need help taking care of guinea pigs from a parent, or maybe yourself. They are definitely lovable, but they are a lot of work if they are cared for properly.
Guinea pigs can be very costly. I found this out the hard way. I would not give my boys up for anything, but I am going broke!
Petstore guinea pig pellets are usually not good. Avoid colored bits and seeds at all costs.
The best pellets are from www.oxbowhay.com
Hay is also a must. Alfalfa is best for piggies under a year, and timothy is good for full grown piggies. A good, quality hay is important. Try oxbow hay (link above) or http://www.kmshayloft.com/index.html
The shipping is worth it!
They should have fresh hay, pellets, and water 24/7.
Then, twice a day you should feed veggies. No iceburg lettuce! A good list can be found here: http://www.guinealynx.info/nutrition.html#fave
Oh! Read this!! http://www.guinealynx.info/healthycavy.html
For cages, bigger is always WAY better! www.cavycages.com
These are the best cages ever! So easy to make, so easy to clean, and very cheap! Read everything on the site!
Also, piggies really shouldn't be alone. They are social creatures, and really enjoy the company of other piggies. BUT!! Definitely get a same-sex pair!! There are lots of homeless guinea pigs everywhere, and you don't want to add to the problem by making more babies.
On that same note....RESCUE!!! Try www.petfinder.com
Or check your local shelters. Please save some homeless babies if you are going to get any!
Some other good sites are:
These people know their cavies!
Also, be prepared to have some major vet bills if you get guinea pigs. A lot can go wrong, but I have been lucky so far. But I have a vet fund (over $500) in the bank put away for emergencies. But that really goes for all animals.
Also, guinea pigs will live for up to ten years. The average is probably 6-8 years. Will your sister still want them when she is 20? You really need to plan for the future with guinea pigs.
Do lots of reasearch with her so she knows about the responsibilities.
Above all: Don't suprise her!!! Pets as gifts are not a good idea!