I certainly do not envy your position. However, I can relay to you a personal experience. I am a JW; my ex-husband was never a JW. He didn't hate JWs when we were married; however, afterwards he hated everything I liked, and this spilled into his relationship with our daughter. She was five when we divorced. He was constantly taking me back to court to stop her from attending assemblies, etc., and I let him! People at the kindgom hall thought 'I should fight' more. But, I knew I was in a battle for her heart, which is emotional, not her brain, which is logical. I knew that children started making judgement calls regarding their parents behavior in the early teens. I was confident that upon reaching adulthood, my daughter would look back and make a judgement call about who caused her the most pain. I didn't want to be the one who caused her emotional pain by dragging her father into court repeatedly.
I was correct. She is now almost 21, and has not spoken to her father for over a year. He won every court battle, but lost his daughter. She is not a JW, but is a happy, competent, contributing citizen...which is just what I wanted, and we have a fantastic relationship. I am taking her to a spa for her 21st birthday. Interestingly, a pioneer sister was asking about her a few weeks ago, who commented that my kids always seemed the most balanced in the congregation.
You have been presented two options here: One is to drag it into court; the other to wait until she can process the information. Should you choose to take it to court, you most likely will not have the outcome that Mary presented as you are dealing with the mother, who has more legal rights, not the mother-in-law. I do agree with Mary that it is emotional abuse, but probably not to the level a judge will intervene.
Waiting until the child is older and can process information has worked well for me. But, each circumstance is different. Focus on preserving your relationship so that you two are still speaking when she is old enough to process the infomation. It would truly be a shame to win your court battles only to lose your daughter.