While I certainly agree with others that this individuals behavior is reprehensible and not to be condoned but I can see some other problems here perhaps that may be in your lap.

First and foremost in both full scale training as well as model flight training trying to attempt training someone who is close family in a high percentage of cases most often results in failure for any number of complex reasons that often are not related at all to the actual flight training.

This could even be worse in the father in law / son in law relationship. If you continue to try to 'instruct' him things will only get worse.

A seasoned impartial instructor would have realized this impending disaster and would have been aware at how inappropriate the choice of trainer for him is. Just because you or someone else may have successfully used this airplane for a first its obviously is an extremely poor choice for him.

His best chance for success is to get involved with a club or at least other mentors and get an experianced impartial trainer to aid him and a genuine trainer type airplane whether it is electric or glow. If he won,t do that then his chances for success are slim to none and possibly your relationship with him.

Just an opinion

John