Complex sentence: Marge stays for a while at Ripley's rented house in Venice; when she discovers Dickie's rings in Ripley's possession she seems to be on the verge of realising the truth.
Split into simpler sentences: Marge stays for a while at Ripley's rented house in Venice. When she discovers Dickie's rings in Ripley's possession, she seems to be on the verge of realising the truth.