Just my opinion but...
It's really not your place to be criticizing their parenting. Her mother and father are the ones to deal with the issue, and they seem to be dealing with it in their own way. He isn't listening to you anymore because he probably feels you are overstepping with this.
Check out the single parent's forum here and you'll see how upset and frustrated these women are when the stepmother is criticizing their parenting. I don't think you would appreciate it if she started criticizing you for your parenting when you have your child.
It's really not your place to be criticizing their parenting. Her mother and father are the ones to deal with the issue, and they seem to be dealing with it in their own way. He isn't listening to you anymore because he probably feels you are overstepping with this.
Check out the single parent's forum here and you'll see how upset and frustrated these women are when the stepmother is criticizing their parenting. I don't think you would appreciate it if she started criticizing you for your parenting when you have your child.