I can understand that sometimes she feels stressed but she does vent to me a lot. Her husband was recovering from cancer (he's in remission now, he's lovely, we get on really well) and she kept coming round here complaining that he was too tired to do anything and he snapped at her a lot and she wanted to leave him.
I had no words for her at that time because I just thought she was being very selfish. I can understand how looking after someone who is poorly from cancer can be very draining physically and emotionally but threatening to leave?
I don't care how poorly my DH gets, I would never ever leave him. I'd want to be there for him and nurse him and get through it together. Not jump ship when shit got rough.
I'm not going to assume I'll cope without any unexpected reactions if my husband ever gets seriously ill. I've discovered that while I am really really good at handling things very well when it's theory, I'm usually less heroic and textbook "doing it right" when the shit does hit the fan.
She brought her pain to you when brought low when taking care of ill family. You brought your pain to her when brought low when taking care of ill family.
Neither of you gave the other the sort of support SIL seems better equipped to provide.
It might be tit for tat. But equally it might just be that you two can't do that for each other. Even if you can for other people.
Rather than shut her out entirely and turn this into a thing that might mar the realtionship and the family dynamic on a larger scale, perhaps both of you need to go to the people who can provide the needed support. Would it be possible to restrict your and MIL's communications to the stuff both of you feel more comfortable dealing with ?
If you have a private chat space with SIL, the group chat can be kept for lighter, happier stuff. So nobody gets a metaphorical slap in the chops due to being cut fro the group and this event won't escalate when 2 women, both at the end of their tethers due to the realities of ill health, need more stress and strife like they need holes in their heads.
I have just the one child. When he got ill, especially the couple of time he ended up in hospital (nothing gravely serious) I was brought to my knees. It was exhausting, stressful and at the time it feel never ending. The thought of that scenario doubled ... it has to be really rough.