I suspected the mother too.
However, she:
Is in Ohio, a thousand miles away
Doesn't know where her daughter lives
Doesn't have enough money to hire anyone to do the job
I agree the girl must have gone out herself. It's a mobile home with paper-thin walls, and the dog didn't bark.
But if she went out by herself, why haven't they found her yet? It's not like there is a murderer lurking under every tree.
The dog didn't bark...