According to the arrest affidavit, the mother told the police the stepfather grabbed poor Riley by the hair and threw her across the room. She either hit the wall or the floor hard, and sustained a skull fracture, which was fatal.
The two monsters are only charged at this time with injuring a child, but I understand the case is going to the grand jury, so I would expect an indictment in the near future. Proving an intent to kill, which would be necessary for a first degree murder conviction, might be tricky.
The death penalty would be too merciful for these two fiends, IMO.
In my mind the mother is the bigger monster. She stayed with her daughter’s murderer and protected him.