Unfortunately, being sisters doesn’t automatically mean a natural bond. First, their behavior is not unusual – their teen hormones have kicked in and it’s expected behavior. It will get better about a month after the spay, so keeping them separated is the thing to do. Keep a spray water bottle handy and some gloves if you give them little sessions together and they start to fight again. You’ll want to be able to separate them fast.

The mounting is dominance, that’s why it’s not only male/female gender behavior. One teen is trying to assert herself as the dominant one of the pair, so that’s what that is all about. Ultimately the bond will work when one decides it’s OK to let the other be dominant. Sorting that out between them will be less aggressive after they are spayed.

Read the bonding section here, but mostly hang on until the spays. If you can move up the date sooner, and your vet feels one or both are old enough, I’d get it done asap. Otherwise, it sounds like you’re set up well enough to get through these next weeks. It’s going to pass!

oooh hormone time.
yuck! time to be spayed and rebonded!

Once the buns are spayed, you will most likely have to start from scratch and re-bond them.
Good luck~

Yes – I agree that it is time to get them spayed. Also, are you sure that they are sisters? Sometimes when they are young it is really hard to tell, and depending on how reliable the person was that you got them from, they may have mis-sexed them. I think that spaying and then starting the bonding process is the best idea.

Welcome here John! It’s great they are going in to be spayed soon! It is necessary, not only for their health, but in order for them to live together again. Let us know if you need help with anything else like pre/post-op care or rebonding them after the fact.

Is that something your vet recommends – medicine via water bottles?  It really wouldn’t ensure that they get the meds as they can go off food and water.  A spay op is pretty invasive and females recover more slowly than a buck may from a neuter. Generally, you should be given pain meds to take home that are given by mouth via a syringe (like baby meds.)  They main things post op is to check their incision, prevent them pulling stitches, pain management and ensure they start to eat and drink so as to prevent a gut slowdown. 
This is from the House Rabbit Society on per/post op care.
 

What pre- and post-operative care should one give?
Give the rabbit acidophilus for a couple of days prior to surgery, just to be certain that the digestive system is functioning in fine form. Don’t change the diet it any way during this time.
After the surgery, continue giving acidophilus until the appetite has returned to normal.

Inspect the incision morning and evening. After a neuter, the scrotum may swell with fluids. Warm compresses will help, but it is nothing to be overly concerned about. With any sign of infection, take the rabbit to the veterinarian immediately.

Keep a newly spayed female away from all male rabbits (neutered or not), as serious internal damage can be caused if a male mounts her.

After surgery, keep the environment quiet so the rabbit doesn’t startle or panic, don’t do anything to encourage acrobatics, but let the rabbit move around at her own pace– she knows what hurts and what doesn’t

Some veterinarians keep rabbits overnight. If your veterinarian lets you bring your bunny home the first night, note the following:

Most males come home after being neutered looking for “supper”– be sure they have pellets, water, and some good hay (good, fresh alfalfa is a good way to tempt them to nibble a bit)

Most females want to be left alone, are not interested in eating at all, and will sit quietly in a back corner of the cage (or wherever in the house they feel they will be bothered the least)

The following morning, or at latest by the next evening, it is important for the rabbit to be nibbling something. It doesn’t matter what or how much, as long as she is taking in something, so the digestive tract won’t shut down. If she isn’t, tempt her with everything possible, and as a last resort, make a mush of rabbit pellets (1 part pellets, 2 parts water, run through blender thoroughly, add acidophilus, and feed in pea-sized bits with a feeding syringe through the side of the mouth)
Occasionally a female will pull out her stitches. Get her stitched up again, and then belly-band her by wrapping a dish towel around her whole middle and binding that with an elastic bandage wrapped snuggly over it. If she can breath normally, it isn’t too tight.