MIL has anxiety. She worries about most things but the health of her grandchildren is a big trigger. Once DD was sick while we were visiting her and MIL didn't sleep at all, just led awake crying and panicking that she had something serious. DH and FIL deal with this by shielding her as much as possible from anything that might cause anxiety. If one of the kids has so much as a temperature we aren't allowed to mention it.
We found out that DD has a small hole in her heart when she was 5 days old. DH decided not to tell MIL as the hole caused no issues and it was expected that they'd just monitor it as she grows up and no treatment would be required. DD is now 2.8 years. She had a routine scan a few months ago and the hole is now causing a valve to leak and it needs to be closed. She is having open heart surgery in 3 weeks.
FIL and DH don't want to tell MIL about the surgery until after she has had it and is out of intensive care and out of danger. I feel that this is ridiculous and that MIL should know that her much loved grandchild is having major surgery. All of us are anxious and terrified, that's inevitable. MIL is a grown woman who is being treated like a child.
I won't tell her if they don't agree, and I don't think they will. But isn't that crazy? If you have anxiety, would you prefer not to know? DD is only 2 but she's bright and articulate. I'm going to start talking to her about the surgery very soon so there's a risk that she'll just tell MIL herself at this rate!