It is amazing to me that a person walking away from a painful or abusive relationship is suddenly the abuser, because they are being passive-aggression by not "working it" with you.
Your daughter and sister have just opted to have peace in their life. They are not required to explain anything to you...they both have a right to enjoy a life free of people that hurt them - period.
This essay almost explains why they would want that freedom. Maybe if you re-read it you will see why they might have opted out of your life...it is clear to me.