She doesn't realize that because up to this point she's been calling all the shots and you've gone along with it. She's got her plan with how things are going to be and she hasn't deviated from it. If you read DB/DR, most WAS's have been planning this out for a long time. It just seems sudden to the LBS because they weren't aware and it takes them a long time to catch up.
Her (or MIL's) response doesn't surprise me a bit. I warned you that as soon as she read something she didn't agree with she would tune you out. Worry about birthdays later, focus on equal time with your kids now. She's in an entirely different place than you and I fear legal action is your only option.
Everybody hurts. It's part of life. Don't miss the good stuff.