ST. PETERSBURG — Detectives arrested a man Tuesday who they say killed a 68-year-old man and badly injured his sister in a Coquina Key home over the weekend.Otis J. Henderson, 44, was arrested about 11:45 a.m. near the intersection of 15th Avenue N and Dr. Martin Luther King Jr. Street.Investigators said Henderson attacked Paul Dumas and his sister, 63-year-old Elaine Vidinha, on Saturday at a home at 4245 Elkcam Blvd. By the time St. Petersburg police officers found the pair Monday morning, Dumas had died and Vidinha was badly injured. She was taken to Bayfront Health St. Petersburg in critical condition.Henderson and the victims knew each other, police said. Officers said they argued before the attack. Henderson faces one charge of first-degree murder and one charge of attempted first-degree murder.