I'd be inclined to simply tell MiL that your family is not welcome at SiL's and that you would not presume to go unless SIL herself decided she would like to invite you. However, if you and your husband both want to give your SIL a second chance, I would suggest that your husband contact his sister to say " Mom has told us that the family christmas party will be at your home, and has asked us to bring ***. We'd love to see you and the kids, but as we haven't heard from you we're not sure whether you'd like us to be there. Could you let us know whether you'd like us to attend, and if so, would you like us to bring [whatever MiL said] or is there something else which would work better for you."
It gives her the power of saying yes or no, and also acknowledges and respects her position as hostess.
I would also say to MIL that you would love to attend the event, but that as it is at SIL's home you won't be attending unless SiL invites you, and that DH will be speaking to her directly. You can make clear to MiL that you are *not* asking her to intervene - indeed you'd prefer that she didn't, but that you can't go where you have not been invited. If you think it would come bettter from your Husband, then he can be the one to talk to his Mom.
Good luck. (can you invite everyone to yours, at a time that doesn't clash with the main celebration, so you get to see the family, even if you don't go to SiL's?)