title: How to define sister predicate in Prolog
body: <p>"Using the predicates <code>parent(X,Y)</code>, <code>male(X)</code>, and <code>female(X)</code>, write a
  Prolog predicate that defines <code>sister(X,Y)</code>."</p>
  
  <p>I am trying to teach myself how to use Prolog for giggles, and the lesson I am using has this as one of the assignments, I do not even know where to begin with this. </p>