She's intentionally not getting it. She's not dumb, she's passive aggressive. She'll get it once the baby's born, but butchering it now is her way of letting you know just how much she dislikes the name.
I would be tempted to tell her "Her name is Ravenna. It is pronounced ruh-VEN-uh. We are not changing it. Either you respect our decision and you call her by her correct name, or you lose time seeing her."
Tough, but hopefully she will get it. She had her chance to name kids. She is completely disrespecting you. She needs to grow up and call you soon to be born daughter by the name you and your DH chose. Hopefully she will not act like such an ass after her granddaughter is born.