Complex sentence: Katy's mother Angela witnessed the kill but persuaded her not to call the police, and they went through a pretence that Tommy had been killed by an intruder, until forensic evidence led to Angela's arrest.
Split into simpler sentences: Katy's mother Angela witnessed the murder but persuaded her not to call the police. They then went through a pretence that Tommy had been killed by an intruder, until forensic evidence led to Angela's arrest.