What happened to the little sister?

The top of her face is briefly seen when the parents and firefighters show up at the end. She and Billy (her other brother) run to either side of the mom and hug her. It was an easy miss, it was maybe a 2 second shot.

I wondered that too - i assumed she died in the fire and everyone forgot that she existed, and the mom says that the son survived was all that mattered! Very odd, I was looking for her, but I have only seen it the once.

She was a ghost the whole time.
Plot twist

When Matt (possessed by Jonah) pushed Wendy and Billy out the house before the fire, you could also see Mary standing by Wendy and Billy. So, it's clear to assume that she got out safe with Wendy and Billy.