My recommendation is to have both spayed– this is not only crucial for longevity and for their individual behaviors, but also because regardless if they are related or not, a pair of rabbits where at least one is intact (i.e., not spayed/neutered) significantly increases the likelihood that they will not have a stable, long-term bond. Young rabbits will get along with most other rabbits due to lack of hormones; however, this changes when they begin developing hormones and will have the territorial/mating/dominance urges that causes rabbits to fight (again, regardless if they are related, raised together, etc…. rabbits do not acknowledge family units or “childhood friends”). It is called the baby bond phenomena. If you intend to keep both rabbits, both should be spayed to increase the chance they’ll have a stable, adult bond when both are grown.
There will likely come a time when you need to house both separately for their safety, as the baby will not be able to be spayed until 5-6mo old. 2.5yo is not an old rabbit by any means, so unless there’s something else going on, post-recovery from her most recent litter, she should be fine for a spay. Consult with your rabbit-savvy exotic vet to ensure all rabbits are healthy enough for a spay.
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.