Well I got an apologetic email from SIL (apologising for both the email and not giving a date for dinner), reiterating that they'd like to have us over to their place (although not specifying anytime soon!).

I guess I should be thankful for at least the fact it appears she is definitely on my wave-length and I can get along with her. We can't 'manage' our emotional other-halves, but what do you do!