NN, wasn't the letter your stepdaughter wrote something of a 'sever all ties' letter with her Mother? Wasn't no further communication from her mother the desired conclusion to their toxic relationship?
I would drop the matter entirely. By you getting involved with her mother re: the letter serves no purpose whatsoever that would benefit any of you.
Maybe therapy sessions with a professional (Psychologist or Psychiatrist) would be the next step for your SD to begin the work needed to get her to a healthy place regarding the abandonment she deeply suffers by her mother.