I might agree with it being a waste of time if the parents didn't care about it. HOWEVER, the OP knows exactly how much it means to her parents and he refuses. Dick.
Why the hell would you pop in when her sister is being yelled at by her parents anyway? Sounds stupid as hell.
"Your husband didn't ask us permission! What a jerk and a terrible man!"
"Oh hey, I know I am not a part of this conversation but just thought I would let you know, I don't want to either. What do you think of that?!"
Yeah it was probably improper timing to bring that up. They most likely wouldn't of said anything if they were calm, they were probably just blindly speaking on rage.