Harper,
What struck me after reading your post was that the roles established in our biological families are very hard to break out of. Usually it takes a lot of therapy, and support to break free.
I personally think it would help your wife more if you were understanding and sympathetic about how hard it is to put the past behind her.
Yes, Mya is at risk here in this situation. Maybe you could hold Mya when your wife is working through her pain in a public manner. Maybe you could say to Mya, "Mommy is processing something that happened to her. Don't worry it will be all right. Mommy is strong and she will be all right. Don't worry. We both love you very much."
EJ