code: public Set<Category> getSiblings() {
          Set<Category> siblings = new HashSet<Category>();
  
          // add siblings
          for (Category parent : this.getParents()) {
              siblings.addAll(parent.getChildren());
          }
  
          // remove this category from list
          siblings.remove(this);
  
          return siblings;
      }
docs: Returns the siblings of this category.
  @return Returns the siblings of this category or null, if there are none.