Members of one League may not Raid and Besiege each other’s Castles. However, they can cross swords on neutral territory, such as a Hamlet or a besieged Castle of some random player.
For example, you send reinforcements to the Castle, and while they are en route, your League mate besieges it. Then your troops arrive at the Castle and start fighting on the side of its owner, possibly liberating it from the Siege of your League mate.