Complex sentence: Jerothe's sister had been dating Tommy DeSimone, following a break-up Tommy had assaulted Jerothe's sister, provoking Jerothe to threaten to murder DeSimone.
Split into simpler sentences: Jerothe's sister had been dating Tommy DeSimone. Following a break-up, Tommy had assaulted Jerothe's sister, provoking Jerothe to threaten to murder DeSimone.