Not enough information to say if this is parental alienation.  Although if the grandmother is behaving in the manner described she is acting inappropriately -- no matter what label you put on it.

In parental alienation one parent damages, and in some cases destroys, a child's normal, healthy relationship with the child's other parent.  Unresolved emotional issues drive the alienating parent to pull the child into the adult conflict and choose sides.  The result is a very unhealthy, co-dependent relationship between the alienating parent and child. 

For more information on parental alienation you can visit http://www.afamilysheartbreak.com. 

Good luck!