Does the adoptive mother sin by preventing her adopted daughter from visiting her biological mother, and is the girl's failure to visit her biological mother considered disobedience?

You must obey your mother in visiting her and staying with her; for the mother's right is great, and her dutifulness and obedience to her in what is good are among the most obligatory duties and the best acts of drawing near to Allah. As for the woman who raised you, she has a right over you, but it is not permissible for you to obey her in forsaking your mother's visit; for cutting ties with your mother is an act of disobedience. Explain to her gently that honoring your mother is a duty that takes precedence over obeying anyone else.