Beatrice Godin died Tuesday night after being in critical condition since the weekend.
She was one of the two daughters of Martin Godin, who police say is the prime suspect in the deaths.
Provincial police say the 54-year-old man was taken to hospital Saturday night after the slayings of three people, including his other daughter, 13-year-old Medora.
The other people who died were Godin's ex-wife, Nancy Samson, and her boyfriend, Benoit Daigle.
Police say the father of the two girls, who is still in serious condition, may have tried to take his own life.
The deaths rocked two small towns in the Chaudiere-Appalaches region south of Quebec City.
Daigle, 39, and Samson, 44. were found outside a home in St-Croix.
The two girls were discovered in a home in St-Isidore, about 70 kilometres from where the first two bodies were discovered.
Godin was found in the same home.