Complex sentence: Tommy suddenly realises no one inside the house actually saw Gilda enter--they only heard her--but prior to that the two women already in the house were in the kitchen, where they couldn't see or hear anyone entering with a key.
Split into simpler sentences: Tommy suddenly realises no one inside the house saw Gilda enter--they only heard her. Before that the two women already in the house were in the kitchen, where they couldn't see or hear anyone entering with a key.