Anne Giddings, 57, lost her younger sister, Jill Foster, 49, and her teenage niece, Kirstie Foster, in the incident last week. In an interview with The Daily Mirror, she described her brother-in-law, Christopher Foster, 50, as a coward who cared only for money and control.
"Chris was completely power mad," Giddings said. "He always had to belittle everyone else. He was a control freak. He shot his pet dog in the head because he couldn't control it -- that is how much of a control freak he was."
Foster made a fortune with a company manufacturing insulation for oil pipes but had been in financial trouble for years. He was about to lose his mansion in Shropshire at the time of the killings.
His mother, Enid, 78, released a statement saying she was unaware of his financial difficulties.
"Chris talked to nobody, we knew nothing about his financial situation and it's come as a tremendous shock," she said.