Sany - tragic situation!
Suggestion: Don't send an enveloped letter to daughters 1 & 2 (betrayed by their mother - they believe - or by their trust in their mother - you currently believe). Send a postcard - message highly visible, easily legible, with the following:
"Matthew 7:9-11
Or which one of you, if his son asks him for bread, will give him a stone? Or if he asks for a fish, will give him a serpent? If you then, who are evil, know how to give good gifts to your children, how much more will your Father who is in heaven give good things to those who ask him!"
Dear [daughter], as you know, I have always given you the best "bread" and "fish" that I knew to find - at any given time - from your infancy!
Do not imagine that I would offer you anything less than the best 'nourishment' that I can find, even now, when you are an adult... but still my child!
If I had filled your childhood's hunger with white bread and fishsticks, but discovered that I was mistaken in my understanding of nutrition, pride would not have stopped me from admitting my error, and correcting your diet - with whole grains, vegetables and fresh foods... such is a mother's love.
I have not changed."
Maybe, you can capture their feel of the natural bond, and hook their curiosity to hear the rest. Or, at the least, to hear you out.