call_instance.rb

module CallInstance
  def call(*args, &block)
    new(*args, &block).call
  end
  alias_method :[], :call
end

usage.rb

class SiblingModuleName
  extend CallInstance

  def initialize(module_name, sibling_name)
    @module_name = module_name
    @sibling_name = sibling_name
  end

  def call
    parent = parent_module_name()
    if parent
      "#{parent}::#{@sibling_name}"
    else
      @sibling_name
    end
  end

private

  def parent_module_name
    @module_name[/\A(.+)::[^:]+\z/, 1]
  end
end

p SiblingModuleName['Net::HTTP', 'FTP']       #=> "Net::FTP"
p SiblingModuleName['Net::HTTP::Get', 'Post'] #=> "Net::HTTP::Post"
p SiblingModuleName['Object', 'String']       #=> "String"