I'm going to go against the grain here, and say that I think the OP has been doing a great job at moderating the reaction to this silliness. I think inviting them to their big do-s and keeping up the cards may be the high road, but there's no point in wrestling in the mud.
I do think that a frank discussion with the SIL is in order though. Sure it's come to abit of a head by having the family celebration scheduled at her house, but it is a long time coming to say, "Look, I know that you were disappointed by our homeowner decision, but our finances are our own private business, and we would like to move past that to mend the family."
Make it clear that, as you said yourself that you're not looking to place blame or recriminations, just put the past two years behind you and move on. Your own words on the subject are natural and true for you, so use them.
After you've made that clear, you can say that you have been issued a second hand invitation to xmas at her house, and you would prefer to celebrate in a home where you are welcome, and now it is up to her to take your hand extended in peace..or not.
I do agree with everyone who says that unless SIL makes a direct invitation for the entire family *specifically* including you, that none of you should go to the celebration.