Your admission in a school is, of course, something positive, which can help you hinder your deportation, but the problem is that you haven’t started your school and it is in a different city. However, it may still be helpful. You should definitely inform your lawyer about it. Due to rejection of your asylum application and the Dublin Regulation, it will be very difficult/almost impossible to give you the permission to move to another city. The process of moving to another city of Germany is anyways a long-lasting process, in which both your respective Foreigners‘ Office and the Foreigners‘ Office of the city you want to move to, are involved. In order to check if you can move to your sister, the Foreigers‘ Office may, in addition to your application for movement and your admission letter, also ask for a right of custody (Sorgerecht) of you being transferred from your parents to your sister.
You should also discuss about your movement to your sister, its possibility and its effect on the asylum application of your parents with your lawyer.