Can CPS keep you from leaving the state if there was nothing wrong with your case?

The case is still open. This is for my sister. She and her husband moved to Alabama with his mother. Well they have a 6 month old daughter and are about to have their second child in about 3 months. The thing is that my sister is bipolar and is unable to be on meds for it because she is pregnant. Well, her mother in law has been giving her some problems since they moved in with her and so they were planning to move back to live with our mother. They already had a child welfare case open on them here in our town from when they lived in Greenville and moved from there to Vardaman. The case was opened then because my sister was admitted to the hospital in the mental ward for having panic attacks. Well the caseworker they had said everything was fine and they were doing great with the baby. So when they told the mother in law that they were moving back , she called child services and had them come out, when they came out she told them that she was going to file for custody. Does she have grounds to do so? If so what can my sister do to keep her child? The child is being well taken care of and is loved by both of her parents. Does a grandparent have the right to take a child like this?