Adoption – yichud

Igros Moshe (IV #64.2) Question: Is there a problem of yichud with adopted chilfren? Answer Raising orphans or adopting children is a big mitzva. And it is common that the adopted children are not aware that they are adopted until they are adults. It is in fact prohibited to never reveal to them that they are adopted because they might end up marrying prohibited relatives. There is no problem of yichud with step children (children of the wife from first marriage) as long as the wife is alive.  One should not rely on the fact that the adopted child has been treated all her life as a real daughter. 

Igros Moshe (IV #71) Question: Is it appropriate to marry a woman who already has a daughter because of the problem of yichud? Answer It happens everyday that a man marries a woman who has a daughter. Since most of the time his wife is with him there is no problem of yichud, even though it happens on occasion that the wife will leave the house for short times. It is necessary to make sure that she doesn’t go for a long period of time. As long as she can return at anytime there is no prohibition. At night if she needs to go to a different house, her daughter should go with her.