S.O.
You don't!! That was easy---well, not really.
You should not tell your MIL anything about this. Your HUSBAND has to talk to his mother.
"Mom, I know you are excited about helping us and we would love your help. But the rest of the family will have to come down to visit later after L. and the baby are home, healthy and settled. We will plan a celebration later that the whole family can come down for, but for delivery, Mom, I am asking only YOU to come down and help while we are at the hospital."
That should do it.