Must I Tell my Child that he is Adopted?
If someone is considering adopting a child, it cannot be stressed enough how crucial it is for them to discuss the halachos involved with a competent Rav in order to determine what the permissible options are. There are quite a number of halachic issues that need to be addressed. For example, if the child is not Jewish, it must be ascertained that the conversion process is conducted according to halachah.
With regards to a Jewish child, one of the issues are: Halachah mandates that they be told very early on that they are adopted—lest they believe that their adoptive parents and siblings are their biological family. Being ignorant of their status would likely result in the violation of the halachos of yichud (seclusion with someone of the opposite gender) and chibuk v’nishuk (hugging and kissing a member of the opposite gender), which may be permitted among biological family members but are forbidden after a certain age if the child is adopted. There is even the remote possibility that a biological brother and sister might marry each other if they are unaware that they have been adopted. #444
ראה במכתב להנהלת אוצר הפוסקים - נדפס באוצה״פ ח״ט ע׳ קל, ובאג״ק חכ״ג ע׳ שט. וראה בשאר המכתבים בענין זה שנדפסו בשלחן מנחם אה״ע סי״א. ושקו״ט בזה בהתקשרות גל׳ תקנא וגל׳ תקע. וראה בארוכה בס׳ נחלת צבי (פעלדער). הנסמן בס׳ על בן אמצת לך (דבורקס). ושם - עוד כו״כ פרטים בדבר.