That statement isn't as definitive as you're reading it.
"My parents bodies were identified but not my sisters, but she was sitting between them with my Mom holding her in her arms. That doesn't leave much room for doubt."
Yes, that could mean there were three bodies found like that, but it could also mean that she was in a middle seat between then and that's where she would have been if she died in the crash.
Also, if that is how three bodies were found, it leaves open the option for the child body to not actually be her, and the body to have just ended up there in a crash (And don't bring up "percentages" or "chances" -- it's a plot point, not a statistic).
As
FatGiant says, it's deliberately ambiguous as a plot point, even if its never going to be acted upon.
Moon's already teased us once, implying that even if MC has logically accepted her being dead, emotionally he's still clinging to that sliver of hope, which may also be something Moon exploits in the future (although I doubt it'll be in a negative way in the long run.)
Anyway, it's just one of many threads dropping from Moon's manipulating fingers into our desperate appetites every few weeks. I'm all for it.