It’s good that you got the bunny out of a bad situation and into your house. If these bunnies are 3 months old, the people who you work for need to separate them now, otherwise they are going to be faced with multiple litters. Some people don’t realize that rabbits can start reproducing as early as 3 months of age. Also, when rabbits reach that age and their hormones come in, the will likely start fighting and can inflict severe injury on each other. It doesn’t mater if they are siblings. I don’t know if that family would be receptive to your advice, but they need to know some facts about rabbits and they also need to know basic care. For rabbits to be able to live together, they have to be spayed and neutered and go through a bonding process after their surgeries.
Keeping a rabbit in an x-pen with a few hours of out of pen time each day sounds like a pretty good set up. It sounds like she would have the room to herself, so that the dog and cat couldn’t be around her unless you are home. That is the way that a lot of rabbits are kept. Since your rabbit is so young, go slowly with feeding her greens. There is a lot of good information on all the basics of rabbit care on this site. Click on BUNNY INFO at the top of the page. You mentioned having a salt lick. Rabbits don’t need those.
Are you sure this rabbit is a female? It can be hard to tell. Even vets have been known to mis-identify young rabbits. If it’s a female, she should be spayed to prevent uterine cancer, which unspayed rabbits have a very big chance of getting. It will also help with litter box habits, and hopefully make it so that you aren’t washing towels that have been peed on any more. If it’s a male, it’s not essential that he be neutered, but unneutered males can spray and may start humping anything in sight, plus they can be very frustrated. If you ever thought that you would get another rabbit as a companion for this one, your rabbit would have to be spayed or neutered as well as the other rabbit.