Just another thought, Mariam. I appreciate your desire to stay with your sister in classes, possibly to help her if she is not so fluent in German. But my guess is, from the director's perspective, he may have seen you as asking for a special favor that he could not in good conscience grant to every student.
Back when I was a university teacher, students would come to me with all kinds of requests for special favours: to do make-up work to compensate for a bad test score, to take a late exam after the regular schedule, to raise a grade to help their graduate school admission, to forgive an obvious case of cheating, and so on &c, &c.
My template was whether I could extend such a favour to the entire class, in order to be fair to all of the other students. Possibly the nursing school director thought that if he made a special exception for you and your sister, he would have to extend special exceptions to your entire class. Probably a lot of students would like to enroll in the same courses as their good friends and family members. But to grant this favor to everyone might make a shambles of their scheduling.
If you were hoping to tutor your sister by being in the same classes, this would be a special advantage for her that other struggling students couldn't share.
Mariam, also, I don't know what is your nationality and socio-economic class. But I have seen international students who (a) either came from more privileged backgrounds and expected the same considerations in the new country, or (b) they came from places where assertive personalities were normal and more aggressive people could prevail by trying to dominate. I am definitely not saying either of these cases apply to you, but if so, just realize that Germany wouldn't operate that way for foreign first-year students. In contrast, you would be expected to show proper deference to authority-- such as a school director.
Even without meaning to appear as aggressive, your significator in your horoscope is Mars, so you probably came across, however inadvertently, as aggressive. But Mars is in its fall, indicating that its power is weakened. Jupiter in Sagittarius, your co-significator, is very strong, but it might come across as expecting privileges, even if this was not your intention.
I do hope everything works out for you and your sister.