Agreed, it's not strictly speaking parental alienation, but it does seem to be similar and related - it's (as described, anyway) the mother in law trying to turn the child against the mother and the father refuses to do anything to prevent it...
I know the father isn't strictly speaking liable for the actions of the mother in law, but I think a practically-minded judge would see that (if proven and backed up by a favourable family report), the father was not acting in the best interests of the children. Also, agree that it's not an easy thing to argue in court and would probably only be resolved after a pretty lengthy battle in which it's not clear what a positive outcome would even be.
I know the father isn't strictly speaking liable for the actions of the mother in law, but I think a practically-minded judge would see that (if proven and backed up by a favourable family report), the father was not acting in the best interests of the children. Also, agree that it's not an easy thing to argue in court and would probably only be resolved after a pretty lengthy battle in which it's not clear what a positive outcome would even be.