I think you worded your message to STBXW beautifully.
I was so sad to read about MIL's treatment of your girls. Such an awful thing to do to your own grandchildren! The more I read on these forums the more I sit in disbelief at the way some people treat others.
I've said it before and I'll say it again. Thank goodness your girls have you. You're doing a great job with them!