While it might be nice to surprise your niece, your sister should definitely be in on it. The last thing you want is to surprise them with pets only to have them not want them. Also your sister should of course have reasonable expectations that she may have to take over certain aspects of their care and be outright in charge of others. If necessary she may in the long term have to take over simple tasks such as cage cleaning, but she should be outright in charge of their diets and medical care (Ie. she should prepare their vegetable salads and measure out their pellets but she can let your niece put it into the cage, she should also be in charge of taking them to the vet and paying for it when necessary).
Also it is important to know that guinea pigs need specialised exotic vets, normal small animal vets (aka cat and dog vets) get pretty much no training in the medical needs of guinea pigs.Meaning taking a guinea pig to a small animal vet is often a waste of time and money and could potentially have fatal consequences if a drug is prescribed that shouldn't be (ie. any penicillin based antibiotic).
Now onto your questions
1. Yes two males can live together, provided they have plenty of space (ideally a 2x5 or 3x4 C&C cage as a minimum), compatible personalities and are introduced properly
https://guinea-pigs.livejournal.com/3002707.html To avoid having to do introductions consider looking into adopting an already bonded pair.
2.
https://www.guineapigcagesstore.com/
3. Bedding depends on what works best for the individual. Options include disposable bedding (such as carefresh) the main con to disposable bedding is the ongoing cost. Then there is fleece with some kind of absorbent material underneath, the absorbent layers that often work best for people are wood pellets, Uhaul moving pads (the recycled denim kind) and towels. A con to fleece is the washing required every week and the initial prepping process (you need to make sure the fleece wicks before it is put into the cage, this can sometimes mean you will need to wash the fleece several times before it wick which is when fluids will pass through the material rather than pooling on the top).
Read
https://www.guineapigcages.com/bedding.htm
https://www.guineapigcages.com/forum/threads/21568-The-Fleece-Project-The-Study
https://www.guineapigcages.com/forum/threads/107419-3-full-weeks-of-wood-pellets
https://www.guineapigcages.com/forum/threads/58123-Review-UHaul-blanket
4. Guinea pigs need an unlimited supply of grass hay 24/7, the most common kind in the US is Timothy hay but other kinds of grass hay such as bluegrass and orchardgrass are acceptable alternatives, particularly if someone in the family has an allergy to Timothy hay. Keep in mind it is much cheaper to buy hay in bulk and those bags sold in pet stores are by far the most expensive way to buy hay. Adult guinea pigs also need 1 cup of vegetables per pig per day, babies need a little less depending on their age, daily staples should include 1/4 of a bell pepper (for Vitamin C, guinea pigs like humans need Vitamin C in their diets to prevent scurvy) and green/red leaf lettuce (not Iceberg lettuce as that has almost no nutritional value and can cause loose stools and gas). A link to a list of vegetables and fruit guinea pigs can have and the frequency at which they can have them is the second to last link in the quote further down.
Adult guinea pigs should also have 1/8 of a cup of high quality plain timothy pellets every day, the brands we recommend are Oxbow and KMS (KMS can only be ordered online from this website (broken link removed) ), guinea pigs under 6 months of age can have unlimited pellets and can have Alfalfa pellets as they need extra calcium for healthy bone growth but Alfalfa pellets are not strictly necessary as you can give them calcium through other means (such as a few sprigs of parsley every day). Adult guinea pigs should have their calcium intake limited as they can be prone to developing bladder stones.
5. I would advise to look into adoption rather than purchasing from a pet store, pet store guinea pigs are often missexed and frequently come to their new owners with illnesses such as fungal infections, respiratory infections or mite/lice infestations. As your sister and niece will be first time cavy parents it is probably best to get an already bonded pair so they don't have to go through the process of introducing two guinea pigs, normal dominance displays when two guinea pig first meet might be distressing for your niece to watch (ie, nipping, mounting, teeth chattering, and squealing).
6. No, the plastic balls are not good for guinea pigs, they can injure their spines in them as guinea pigs have very fragile backs which the ball forces to bend into a dangerous posture. Guinea pigs have also in the past injured their legs and feet inside such balls. Also avoid leads, harnesses, wheels, mineral/salt licks and most pet store treats, all of these things are bad for their health. Pet store treats are often full of ingredients guinea pigs should not have, such as sugar, as it is bad for their digestive systems.
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I know that is probably a lot of information, but here is some more reading material @
bpatters usually recommends for new guinea pig owners
