From what I’ve seen, the lack of seat belt use may not have been a major factor here. One of the five fatalities was belted in, so this was clearly a high-impact collision that would have caused a lot of damage even if everyone was belted in.
The mother (lone survivor) front seat passenger, was wearing a seat belt. I've heard she was asleep at the time of the accident. I think she had a broken leg, broken arm and broken ribs.