Yes, anything is possible and there have certainly been cases where parents or other family have been callous in their disposal; however, it's just much less likely from a criminal profiling point of view. A close family member with a negative emotional attachment would very likely be considered mentally disordered (for the purpose of profiling) and this falls outside the norm. A step parent (or other relative who may have less attachment) I feel would be more likely to go to greater lengths to hide the evidence, as a discovery and ID of the remains would be easier to lead back to them.
The circumstances of the disposal indicate that the person who dumped the remains isn't too concerned about them being found, so it's highly probable (but not definite) that they felt confident an ID
wouldn't lead back to them.
It's difficult at this point, as we are all limited in our knowledge of what evidence has been found, so there's just no way of knowing, unfortunately. Your theories are as good as mine
ETA: Just on this point, if it was a step parent or other relative, the child will very likely have been reported missing by other family members, so an ID will be more likely.
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