People excuse her behavior with a car accident she was in almost twenty years ago in which she suffered major brain trauma.

This point jumped out at me so much. My ex had had a brain tumor years before I met him, and he also blamed his forgetfulness on that. I know, right after the surgery, he was a bit slower, more like his body/brain were in shock. So yes, during this time, his forgetfulness was understandable. He just had brain surgery.

But he kept up. He would forget everything from important dates to remembering to pick something up on his way home. It was so aggravating because he wouldn't let me mad at him, since according to him it wasn't his fault.

Apparently, like your MIL's wife, my ex conveniently forgot about his brain injury in every other aspect of life, because he was fine remembering things his friends told him (that he actually wanted to know/do) and at his job.

Sorry- slight trigger for me, but my point is, if she's fine in every other aspect of her life and is only using her brain injury card to get out of bad or inappropriate things, then she's probably just a bitch in real life and the injury doesn't play a part. It's been 20 years, she should have adjusted by now but has realized she can still pull that card and carry on scott-free.

Have you talked to your husband about this? Maybe set a list of ground rules that EVERYONE at the hospital needs to follow, and make sure they all know if they break the rules, your husband will kick them out. Make it so there can be no discussion. And ~maybe~ make the list in your favor by putting stuff you know your MILs will do that will get them kicked out.

Ultimately, this day is about you and your husband. Maybe just a blanket statement of "This is our day, and we are choosing to spend it with you, our family. But if anyone does anything to cause an argument or create any sort of issue, you will be kicked out."

Good luck, and congrats on the baby!