cherrybim,
You are mistaken. There is no such thing as halacha being something is assur, but that “no Posek will ever tell them that they must adhere to these rules.”
You are also mistaken that “all contact between the father/[adopted] daughter or mother/[adopted] son must cease and Yichud issues apply as well.” The Igros Moshe E.H. 4:64-2 says it is permissible. (Most other poskim disagree with Reb Moshe though.)