Maybe
Uncle Justin came into the livingroom and started messing with Willow (playing with her I mean) like uncles do. He's a tall guy, pretty muscular. It would not surprise me if her death was accidental..."facing" 1st degree murder means he killed her. Without having all the facts of the situation, when someone dies and there is an arrest, that person faces all sorts of varying degrees of murder charges until developments are made and something "for sure" comes of it.
Maybe Justin was just playing with his little niece, things got too rough and her tiny neck broke from a fall after uncle Justin accidentally dropped her while playing the "hold ' her upside down' game. That thought makes me sick to my stomach but it is a possibility. I can see a guy that young freaking out and just grabbing her, fleeing the house and dumping her outside of town. It would also explain why little brother was upset. I've never been in that sort of situation, but everyone reacts to horrific things in different ways, and I can see a "freak out session" on his part and just dumping her to make it go away.
This is all total speculation but I pray this is a situation where accidental is the reason. It would be better than the alternative.

I'm so very sad right now.