How do we tell MIL that her actions have led to her not meeting her new grandchild?

I’m due in a few months and my MIL (and SIL but not to the same extent) has made it her mission to make all of our milestones about her. If the attention isn’t on her, it’s a problem. She’s actively trying to drive a wedge between my husband and I, and without going into all the details…. She’s not stable enough, mature enough, reasonable enough, reliable enough, or respectful enough to be involved in this child’s life. Should the tides completely change (unlikely – seems she’s been like this her whole life), we’ll revisit our decision… but for now, she’s not going to be meeting him.

This is obviously something that’s going to have to be communicated to her but the timing is something we’ve gone back and forth on. She’s not mentally well and although she doesn’t threaten suicide, she does go MIA for weeks at a time and lets everyone think she’s hurt herself. I’m not interested in the dramatics and I guess there’s always a chance she actually does something, but in my experience narcissists are too in love with themselves to hurt themselves for real. I also don’t want to leave this conversation until the last moment and have her meltdown completely overshadow the happiness and experience of being a new parent.

No matter what, there’s going to be a meltdown and world war 3.