Please have your sister immedately seperate the male and female from one another! They can reproduce at a very young age, if in fact the male has not already inpregnanted the female. Rabbits cost more to spay because they are considered exotic animals and can not be handled the same way a cat or dog would be. Many times the female rabbits will go into pseudo pregnancies or heats and that can cause the reproductive tissues to swell and make surgery even more complicated for the vet...so don't be alarmed to see a much higher price for the spaying/neutering of a rabbit vs. a common dog or cat.
Also, while some may say the surgery can be risky- keep this in mind, if she does not have her rabbits altared, the likelyhood of them developing and dying from reproductive cancers is VERY high.
The $300 quote to have them altared is actually right on the money for an exotic. It cost me close to $250 to get my rabbit spayed when I adopted her. The surgery is not risky however when done by a properly trained vet who knows how to handle rabbits.
You need to have your sister call around all of your local vets and ask them how many rabbits they treat in a month, how many they spay/neuter/ and if they're familiar with what drugs should and should not be used on a rabbit. Rabbits are exceptionally sensitive to certain medications that dogs and cats can typically have, so special care must be given that the vet and staff know what to use and what not to use on a rabbit or it could potentially be fatal.
The reason some inexperienced vets say the procedure can be risky is because of the anestesia that is used. An experienced vet will weigh the rabbit and know how to give just enough anestesia (and what kind) to knock them out enough to not feel anything, but still keep them light enough so they'll come out of the anestesia and not die. In the event they do have a hard time waking up from the anestesia, an experienced vet should always know to give the rabbit Yobine (it's a reversal). That should usually do the trick!
Following the surgery, I'd highly recommend that the vet give her pain meds to take home for the rabbit to ease the discomfort post op. Make sure the vet gives her ones safe for rabbits though! (you'd be suprised how much some vets know so little about rabbits!). Just because a vet may be good with cats or dogs does not mean they will be experienced with rabbits. My rabbit went to the most skilled vet in the city when it came to exotics....he was an expert on rabbits in our area and was so nice. My cats and dogs go to different vets though because their vets were not experienced enough with rabbits in my opinion.
When she has them altared, they will need to wear an e-collar and be kept in a single-story cage until the stitches come out, because rabbits are quite hard to keep still, let alone prevent from messing with their stitches. The best way to get an e-collar on a rabbit is to put a regular small dog collar on them then poke holes in a small e-collar on the bottom of it and attach it all the way around the regular collar to secure it. Make sure it fits them properly so they do not get any feet cought in it. (Cut off any excess.) Also, is she keeping them inside or out? Rabbits live much longer,healthier lives if they're kept indoors.
While the vet has the rabbits out under anestesia that is a great time for them to have their teeth examine and filed if need be.
Do you know what your sister is feeding them? They need to be eating a Timothy Hay based pellet, and given fresh fruits and vegetables, salt blocks, and unlimited Timothy Hay daily. Make sure she is not giving them too much alfalfa as that is not good for rabbits to have in excess (if they're babies it's ok for a while...but once they're larger, alfalfa should only be an occassional treat. Timothy hay on the other hand should be offered in an unlimited fresh supply every day to keep them healthy.)
Please feel free to pm me if you have any questions!
(The house rabbit society's website has some excellent info- please have her check it out! http://www.rabbit.org/