Reguardless if same litter that dont mean they will not fight when there balls drop, yes some brothers will get on but most dont, if you were to get there nuts cut off within a week they would be able to go back together again.
Not sure about the one with the jill but could it be he is also just being a horney boy or is it a case of him being pure nasty, What is he like with other jills?
Mind hobs come in to season long before the jills.
he was doing it a long time before his balls came down.
Anti social ferret then, neutering him might help but ddoubt it.