Complex sentence: Clinton sneaks into Berry-Berry's room when his brother is out, and gets the latter's pistol (which Berry-Berry had shown him earlier) and waits behind the curtains for Berry-Berry to return.
Split into simpler sentences: Clinton sneaks into Berry-Berry's room while Berry-Berry is out. He gets Berry-Berry's pistol (which Berry-Berry had shown him earlier) and waits behind the curtains for Berry-Berry to return.