How long have Hazel and Fiver been together, and how old are they? Rabbits don’t actually “do” family units— brothers, sisters, children and parents, etc.. all actually need to be bonded, because they are equally likely to try and kill and mate with each other, as with non-blood related rabbits.
The tension between Hazel and Fiver is probably because with Pipkin present, Hazel and Fiver’s potentially-not-solid-bond is now in being thrown about, as presence of an unfixed rabbit can cause fixed rabbits to act hormonally. The presence of a new rabbit ordinarily would not affect the existing bond of two other rabbits, so it may be worth separating Pipkin from them until he’s well neutered and done with his hormones, and then doing each one individually with each other for safety.
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.