I mean NO disrespect here, but it sounds like the whole family needs to mentally tell the sister in law to buzz off.
In this example I would say the mistake was in listening to her. It would have been better to ask your mom, "hey, mom it was SO nice having you all over this evening and we've REALLY enjoyed ourselves. I know we have plans for Easter, I just wanted to be sure that there weren't any new details I should know..."
That way if she WAS wanting to "uninvite" (way rude in Ms. Manners book) Mon she could do so herself.
One of those details we all talked about on the communication thread... but it's hard to put into practice with people we've had "unhealthy" communication with our whole lives.
I would start practicing it with them though. Never assume ANY single one of them knows what another is thinking.. so if one says another one wants/feels/needs, go back to the other one and verify it. You don't have to start a rumor mill with "so and so said you...". You can just say "hey mom/dad/bro/sil, I was thinking you might like... but I thought I should ask" or "hey m/d/b/s, how do you feel about...."
They may hem and haw, but you will be teaching (through example) much healthier self-responsibility for speaking the truth of what they need/want etc.
AND if your son witnesses your actions it will help him be more confident in using the good communication skills he's learning with you, PolyN and Mon at home-out in the 'real' world too!