Needless to say, the children are also learning-disabled. But I'm still concerned for their future. Their parents are allowed visits under adult supervision, and the girls are angry at their mother, not their father. I can understand your pain and confusion in the face of this sad situation.
Children are often remarkably resilient, even after having suffered isolated instances of abuse.
If removed from the abusive situation and placed in a loving, supportive environment, some such children will probably do fine without therapy. The effects may be delayed by many years, but can appear as behavioral problems in school, social withdrawal, aggressive or self-injurious behavior, depression, substance abuse, etc. The problem is that if your nieces have no motivation to enter into counseling at this time, it may not do them much good. But if the anger you describe seems to be taking an emotional toll on them, counseling may be helpful even now. Perhaps your sister has a better sense of these issues, since she sees your nieces every day.