Ok - I'm sorry to sound cold but I would be enforcing NC with MIL at least short-term since she obviously shows no care OR respect for the boundaries you are trying to put into place for SD's lifelong emotional health. The child's welfare HAS to come first here. What extenuatiing circumstances have held you back from doing this?
That is how I would, beyond a shadow of a doubt, handle this situation. Not saying you should, but I know I would go this route at this point especially with so many failed attempts to ask her to honor your/SD's boundaries. BTDT. Someone said it so well in another post this week - if a stranger treated your SD this way would you continually allow her to be exposed to that person/abusive treatment?
Here's the deal - Yes, she may know what her therapy tells her & Yes, she may be able to recite the serenity prayer backward. At the end of the day, she's a young girl at a particularly sensitive developmental point in her life & she is VERY clear about what upsets her. It is up to the adults to listen & help her enforce. She still obviously has expectations that she can't stop herself from forming - that MIL will suddenly change her behavior, yet she has shown you time & time again that she refuses to. In this case, MIL is showing you exactly who she is by her actions.
I'm sorry, I'm not trying to come off harsh. My heart is breaking for your SD - it is so unfair to be consistently disrespected like this after everything else she has already endured. Good luck no matter how you decide to handle it.