The technique we are using for SIT is pressure and release.
Rabbits do not like pressure being applied to them, they will attempt to release the pressure any way they can. For SIT you need to be a tree, in others words don't move, and use the leash to hold the rabbit in place just in front of yourself. Holding the rabbit in place applies pressure to the bun. Too much leash and the rabbit will just bounce around from one spot to another. Too little leash and you are lifting the rabbit off the ground.
Once the rabbit settles down in a sit position release the pressure on the leash, this is the rabbits reward for sitting, and give the SIT command. Like all training this will need to be repeated many times before it becomes second nature for the bun and yourself.
In the photo Reeses is in the position you will want to use for this work and is sitting with very little tension (pressure) on the leash as it is being released.
Good luck with the pressure and release technique.