Complex sentence: Sickert took a keen interest in the Jack the Ripper crime and believed he had lodged in the room used by the infamous serial killer, having been told this by his landlady, who suspected a previous lodger.
Split into simpler sentences: Sickert took a keen interest in the crimes of Jack the Ripper and believed he had lodged in a room used by the infamous serial killer. He had been told this by his landlady, who suspected a previous lodger.