Cervical stenosis that causes cord compression produces spinal cord dysfunction and the symptoms of myelopathy. The spinal cord is not just “a long nerve” but in reality an extension of the lower part of the brain. Damage to the cord is similar to damage to the brain. This is why decompression of the cord is important prior to the onset of significant myelopathy. Surgery is designed to prevent further damage and recovery from cord injury will not be known for a year.
The cord has functions that involve coordination of the muscles. Your sister recovered her walking ability which is wonderful but she was left with residuals of hand incoordination. Full recovery is not guaranteed. Hard work and relearning through tracts that are undamaged is important. There is no way to know what the final outcome will be.