I think that Cassidy is going through an extremely difficult time right now. She most likely reverted to bed wetting and is in a form of shock and withdrawal (in her knew unfamiliar environment). Knowing what little I do of children that experience trauma at that age, they often lose memories from time prior to the traumatic experience. Cassidy is probably having her life re-written right now. By being deprived of contact with her mom's family, she will be (in my opinion) forever permanently distanced from them. If enough time passes, then any time she sees them, she will somehow associate them with some feelings of the trauma.
I would urge Michelle's mom and Meredith to seek immediate visitation with the argument that it is in the best interests of Cassidy to maintain a healthy relationship with her mother's side of the family, and that maintaining that relationship will ease the loss of her mother. Furthermore, this clearly would have been Michelles wish. I think they need to act right away because the more time that passes, the more likely the courts will buy the argument that it is better to cut all ties until she is well again.
If Cassidy isn't already seeing a therapist, they should also include that request in their documents and if she is seeing a therapist, they should request a meeting with that person to speak on Michelle's behalf.
If they have to turn their lives upside down to achieve all this, they should because they're only going to get one chance and the longer they wait, the slimmer the chance.
