My SIL & BIL travel for most major holidays. My MIL freaks out every time. My fiancé and I get caught in the aftermath. How to approach this?

It's hard to keep it brief but I will do my best. My fiancé's brother and my SIL have a 3 year old son, my nephew. My MIL was a single mom and her grandson is the light of her life; this is her only grandchild. Things have been mostly good now with MIL, but there is some history of her drinking/verbally lashing out and having very high expectations of how often we should all see her. She is in therapy working on all of this and the last year has been overall uneventful (in a very positive way). For extra context: MIL does not get along with SIL's parents. MIL gets along with my parents and my fiancé and I do not have a child, overall our relationship is fairly smooth sailing. It only gets rocky if we get looped into the fallout from everyone else.

SIL & BIL went away for Thanksgiving last year with my SIL's parents, and it caused a nuclear fallout when my MIL was told (she was not invited). As MIL does not get along with my SIL's parents, it makes perfect sense why she was not invited, but she does not see it this way. I think the combo of 1) missing her nephew on a major holiday and 2) being told last minute caused some strong feelings. MIL complained to my fiancé and I for weeks on weeks, and it really put a damper on our holiday season.

I don't blame SIL & BIL for going away, but I do wish they had set things up better by telling MIL with more than a couple of week's notice (to add insult to injury, they asked MIL to watch their dog while they were away. She refused so fiancé and I were stuck with their yappy dog for 12 days...).

Now SIL & BIL are repeating history this year and have planned on another trip for Thanksgiving time, AND also now during Christmas, we just found out last night. MIL has not been told about either of these plans.

My fiancé and I wish we could spend holidays with our nephew, but ultimately I'm not going to get upset if someone wants to do what is best for them. My SIL has guilted me in the past about not seeing my nephew enough and I feel like it is slightly hypocritical since they are the ones who go away all the time (they travel a lot throughout the year in addition to holidays). My fiancé and I had a trip planned over my nephew's birthday party last year and SIL didn't speak with me for weeks (she party she planned was not on his actual birthday date, we didn't intentionally plan over his birthday). We ended up cancelling our trip to go to the party- they really seem to pick and choose which days are important/crucial.

I really just try to stay out of the way and be happy for everyone. BUT is it reasonable for me and my fiancé to ask them to please give MIL a reasonable heads up about the upcoming holidays so she is not caught off guard like last year? I think the 3 of them need to really have a conversation about the holidays so expectations are set. Is it out of line to say anything to them? How do we tell MIL that we don't want to hear about it when she inevitably gets upset and wants to vent about this without looking like bad guys?