No, she can't, unless and only if she is been made your legal guardian. Otherwise, your mother, as parent, is your guardian and has parental rights, unless those rights were specifically taken away from her, such as by some court proceeding (e.g. if she were found to be an unfit parent) or if she had given up the rights voluntarily under some circumstances (which usually only happens in adoption situations). So the real issue is, if you are a minor, who has the guardianship of you is what determines where you should live and who makes the decision as to where you live. Ex-girlfriend of your father, without more, does not give her any rights or say over you. You mother should be able to take legal action, if the ex-girlfriend tries to impede you moving in with your mother. Good luck.