Three people have died and a man has been arrested after a violent incident at a family home in Sydney's south-west overnight.Emergency services were called to Juliet Close in Rosemeadow just after 1:30am on Sunday, where police discovered the bodies of a woman in her 60s and a man in his 20s.A 64-year-old man was also found at the scene with critical head injuries and later died at Liverpool Hospital.An hour later, officers arrested a 32-year-old man who arrived at the scene.He was taken to Campbelltown Police Station.Police have confirmed a man in his 20s also received serious injuries during the alleged assault.He was taken to hospital in a serious condition.Police believe the incident was domestic violence-related, but the direct relationship between the people has not been confirmed.The homicide squad will assist the police investigation as it continues this morning.Police will hold a press conference in the coming hours.