I am so sorry your SD has so much drama in her life, and so grateful that she has you and your husband to trust and look up to.
Trying to reason why anyone else does anything, particular those who engage in addictive behavior, will set you spinning in circles. We cannot control other people, we can only tell them how we feel, and then we can choose whether or not to take their behavior personally. We can only create healthy boundary for ourselves and act upon them when they are trodden upon.
What your SD is enduring is not fair, certainly. All you can do is provide stability she can count on and help her move forward, away from the toxic behavior of others.