Yichud between Father and adopted children?

Igros Moshe (E.H. 04:064.2): Yichud between Father and adopted children? It is a great mitzva for a person to raise an orphan. Also many people who didn’t have children adopt children and raise them and don’t inform the children that they are adopted until they grow up. Some don’t want to ever reveal this. But that is prohibited since it might result that they marry relatives that are prohibited to them. It is also prohibited to conceal from them the names of their biological parents unless the intent is merely to wait until they grow up. At some point it is required that they be informed not only the names of the father and mother but where she is. However in most cases this information is not known by the adoptive parents since the adoption was done through an agency which cannot be believed whether the mother was Jewish. Therefore we need to assume the child is not Jewish and there needs to be a conversion because of the doubt. Nevertheless when this is done properly we still have to be concerned about yichud even though this is often ignored until the age the adopted daughter reaches the age of 8 or 9 since she is being treated all these years as if she is the biological daughter.  Yichud is a bigger problem then showing her affection by hugging and kissing which are not done from lust and are not prohibited (Shach Y.D. 157.10).  It is possible to say there is not a problem of yichud for short periods of time because he is afraid his wife will find out. Therefore if his wife dies he needs to be careful not to have yichud with his daughter or his adopted daughter