Rabbits do not really acknowledge family units, so them being sisters does not guarantee they’ll get along, especially after a spay, where a hormonal spike occurs during recovery. Following puberty and being spayed, it is normal for rabbits to need a bit of a “booster shot” of the bonding process, if not need to go through the entire process. You will find many topics on our forums where two siblings/young rabbits got along fine, and then things go awry once puberty occurs. It also sounds like Stripey may have been a really important part of the trio-social-hierarchy, and now that she has passed, a new dynamic needs to be agreed upon.
If them “sorting out politics” is leading to something dangerous, like escalated fights, one rabbit bullying another, and/or injuries, it is probably best to separate them from each other and go through a formal bonding process. There is really great overview and steps of bonding in the RABBIT INFO section of this website.
Lastly, I’m really sorry to hear about Stripey. You are probably providing a much better space for her children than the school would have ever done, and your concern just shows how much you care and love them.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.