Background: SIL got married recently, and is itching to get us round to watch the video. I have already been subjected to seen it and DP is of the opinion he was there so why does he need to sit through a 2 hour video.
Fast forward to this afternoon. Had a lovely afternoon at MIL's, SIL was there, talk turned to DP's friends birthday on Monday. I said "I really don't fancy it..can't you wheel out the old excuse about it interfering with DD's routine?" I then turned to SIL (who has no children yet) and said "You'd be amazed at the boring shit I've managed to get out of with that one - she's fine having a night out of routine really but they don't need to know that". All jovial and smiley and consipiritorially with SIL at this point.
We left shortly after, and I did think SIL was a bit quiet, then, as we drove off DP informs me that he had, in fact, used this very excuse to get out of going round there tomorrow night for dinner and wedding video viewing yesterday and not told me 
I am mortified, feel awful and genuinely don't know what to say to make it any better....any suggestions?