This is a very hard one. It IS disrespectful, yet you are feeding into his oppositional nature to go against him. Not to sound rude, but is this issue just that your sister is a pot stirrer who treats you poorly or is there some dating history in the deep past between she and your H?
If it is not EA, I would strongly consider backing off on this particular transparency issue for awhile. Try to give yourself at least 48 hours or more until you decide whether or not to open your mouth. But allowing your resentment to grow about it doesn't seem to be a good idea either. Ultimately (in a month or so?) it will have to be discussed again unless you can decide it is not a dealbreaker for you.
