I think assault is to strong a word for what the Thorpe's did to Catherine. James does stand by while they hold her back but the Torpe's are close friends by now. He is college buddies with Mr. Thorpe and is head over heels for Isabella. I think that this close relationship as he sees it allows for this type of disagreement. I think if it was someone they just met or had not been so aquainted with James would have responded to aid his sister. As it is he probably just views it as aquarrel with some close family friends. While this is very upsetting to Catherine it is mainly so because of her attraction to Henry. The other three don't necessarily know how upset she really is and can't understand her lack of cooperation. I am sure the situation appears much more grievious from Catherine's point of view which is all we have than from the other's point of view. Also this trip is crucial to James even life changing for him and he really wants his sister to be a part of the special day but can't explain why without spilling the beans.