The victim's daughter, Diana Torres, described the incident to Orlando's WFTV.
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"It was big. The flames were pretty high and the whole bed was burning. My father was in the bed. It was horrible. I was screaming," Diana Torres said.
Jane Watrel of the Orange County Sheriff's Office said the victim, Santiago Torres Sr., was sick and lying in bed when his son-in-law found his 43-year-old son in the room.
"My dad was burning. He had his back and his neck burned," Diana Torres said.
The son-in-law said Torres Jr. yelled at him to get out of the room.
The unidentified son-in-law later noticed glowing coming from the man's room. He said that's when he saw the pillow and a mattress on fire. He said he found the victim on fire.
"We seen him burning and we went crazy," Diana Torres said. "I pulled him [Santiago Torres Jr.] out because he was hovering over my father. And pulled him out and threw him on the floor."
Torres Sr. is in serious condition with upper-body injuries.
Diana Torres told WFTV that her brother has a long-standing drug problem and was stealing and taking her father's pain medication.
She said he'd been aggressive in the past. She added that she recently had an injunction against him lifted to unite the family – and so he could spend the last few months their father has together.
Diana Torres said she suspected her brother was high on her dad's morphine Tuesday morning.
"I don't think he even realized what was going on," Diana Torres said. "My brother just needs help. He needs mental help."
Torres Jr. was arrested and charged with attempted homicide and arson to an occupied dwelling.
The state fire marshal is investigating.