OP here. Just clearing up some points NyaChan made. No, we usually include MIL in our holiday plans. She is always invited to the ones at our house or at my DS. DS and I take turns hosting. When DS has it, she always includes MIL and DH aunt who lives alone. No issues there. DS would even include SIL even though they don't mix well. Even though DS does really need a filter when she speaks at times, she does have her heart in the right place. She is excepting of anyone the way they are. May not like them but won't exclude them.
We did not ask MIL this year because DH already knew about MIL going up to SIL's. The one time (years ago) MIL declined to come with us to DS house, SIL called up DH at work and blasted him for leaving MIL alone on the holidays....of course she and her own DH were flying off the the carribean (this was before they had their own kids and started incuding MIL sometimes).
But the dog is at our house, they are off to a time share. When they come back, I am going to try to have a talk with my BIL (SIL's hubby) and feel him out to see why SIL is doing this. I would approach SIL herself but I I don't think she would be honest with me if there was an issue nor do I think she would 'get it'. From what I have seen of her arguments with others (especially her own DH) she doesn't often see anyone else's point of view so this may be futile. However, I have seen enough discord among my mother's family and would like to make an attempt to avoid what I fear is coming once my MIL is no longer with us. I think BIL is much, much more reasonable and may even see what I do. Either way, I would like to at least try. DH won't try to handle issues until he is ready to explode which will put SIL on the defensive. BIL and I are the two with the cooler heads. Now we just need the other two to be reasonable.