Half-siblings... it's complicated

My 21 month old daughter has been with us since she was 2 months old. We provide a safe, loving, nurturing beautiful home where she has lived as my daughter and bonded with my 6 year old son all of her life. The social workers told me I had to do sibling visits with her 6 month old half-sibling (they have a bio-mom in common) that she has never met and that the visits will continue for the rest of her life. The guardians of the 7 month old have contact with my daughter's 19 year old, drug-using, mentally ill bio-mom. In writing it sounds mean, but just trying to paint the picture that there are issues with the safety and autonomy of my family. There are also issues with my daughter's self-image, development and the whole self-fulfilling prophecy worry. How confusing to take a child on these visits and tell her that is her "brother". If her permanency plan is to be raised by us, isn't it our responsibility to act in her best interest and have our attorney stop these visits at once? If she is our adoptive daughter, isn't it our choice who she associates with at this point in her development? A seperation of siblings was ordered months ago. I believe a grave error was made and a statute was violated by not placing the boy with us when he was born and they are trying to backtrack and fix their mistake and the best thing they can do is to let these children be! I found A LOT of case law to support my beliefs, but cannot find any psychological studies. My husband jokes and says how can they do a study on the psychological effects on children not knowing somebody they don't know. Any help or support would be appreciated.