gistfile1.rb

module Helper
  def self.parent(scope)
    chain = scope.name.split("::")[0 ... -1]
    chain.reduce(Object) { |m, o| m.const_get(o) }
  end
end

module Family
  def sibling(string)
    Helper.parent(self).const_get(string)
  end
end

module A
  module B
    class C
    end

    class D
      extend Family
      puts sibling "C"
    end
  end
end