The fire erupted before dawn this morning inside a garage apartment on Nancy Drive in Massapequa.
Fire officials say the homeowner tried desperately to save his sister and her husband, but the flames were too intense. The two were trapped inside.
Neighbors say 56-year-old Henry Munsch and his 51-year-old wife Toni were devoted to one another in life and in death. She had Multiple Sclerosis, a debilitating condition, which could have hindered their escape
The homeowner suffered second-degree burns to his face, arms and hands trying to rescue the couple.
A family friend who was visiting from Florida was not injured.
Two firefighters suffered minor burns and were treated at a nearby hospital.
The fire at the 2-story house in Massapequa was called in at 4:06 a.m. and brought under control an hour later.
Police say the cause is not considered suspicious.
Fire Marshal Vincent McManus says two non-working smoke detectors were found in the house.