UML Class and Sequency diagram of Library

So there’s a library to make a Class diagram for.
The classes are

  • Library
  • Librarian
  • Reader
  • Book

but for example when a Reader didn’t bring back a book for a long time, or when a reader has too many books, he can’t purchase more. But how could the Librairian possibly check this (from which existing, or new class he can get that information from). Any help would be appreciated… and the sooner the better. Thanks in advance. (and I’m sorry if it’s a stupid question)