Write a letter to Head of Department at University

Hello! Can anyone help me to write a letter to a Head of department at UK university. The thing is that I have a twin sister. We, both, graduated from the UK university, and would like to continue our education at Master degree. We have already submitted our applications. However, my sister didn't have a few points, more precisely 1.4 points to the rquired one. Thus, I got an offer from the university, but she was rejected because of her grades. In this regard, I intend to write a letter for a head of department in order to convince him to give her a chance. Since, we do not want to split up. And, for our parents it's more convinient if we could study at the same university. Please, I'd be very grateful if anybody could help me write this letter. Thank you.