I understand your feelings, but I'm going to play devil's advocate.
Is your SIL uncomfortable for any reason at your mother's house? Or, does SIL have an elderly grandparent where this could be the last Christmas with them? The latter reason is the reason why we always do Christmas with my family and not DH's. (That and it costs a fortune to fly to Wichita, KS)
I really do understand how you feel. And generally I believe in trying to split holiday's between families. However, some people are just more comfortable in certain situations. I think my DH is actually more comfortable at my parent's house than his own. Maybe because we spend more time there, I don't know.
Have you thought of inviting SIL's family to Thanksgiving or Xmas at your parent's house? If you all get along really well, it could be loads of fun!
All that said, I think that someone should broach the topic with your brother. If it's causing this much heartache, it's definitely not something to let fester. I'm sure your brother doesn't mean to upset anyone.
For your bold part, I don't play that card EVER. Anybody, at any time can pass away. Just because your elderly doesn't trump everybody else's live. I could be gone today by an accident. DH's 80 year old parents could pass away, tonight for old age. You don't ever know. And I personnally know of 3 different families - the wives actually - that have pulled that Granny hasn't been well for several years at the holiday that they are to spend at the other families side. I've told 2 of them, they are selfish and I don't want to hear one word outta them about how selfish their kids are, when they are setting the example from which the kids learned this trait. Once someone pointed it out to them, the finally saw the light and changed their ways. The other one, after the 3rd attempt, hubby set her straight, that is Granny needed her sooo badly, that was fine, but he and the kids would be going to meet the needs of their other Granny.
For the OP, talk to your brother. Let him know what your mother is feeling. It will be up to him to heal the damage that he has caused.