From what little you have I would tend to agree with your sister and your obvious suspicion that hubby was after a bit of extra-marital while you was nicely out of the way - of course to accuse him of that would be to call him a liar which is something you shouldn't really do without positive proof...
It isn't unusual for men (and women) to want a change sometimes and it isn't unusual for them to have a few stupid ideas.
It is possible he was testing the sisterly bond in the time honoured fashion of giving out some false information to test how long it takes to come back. People do this in order to discover who is reliable and who passes on gossip - though what motives your husband would have for doing that is something you can only guess about.
Perhaps it was temporary insanity?
Whatever his reasons it was in very poor taste...
Good luck with the surgery!