The "how" of any scenario involving the mother is the problem for me.
Did she leave her two sons while she ran out to the dump?
Discover that in a big fight or in an overdose, she had killed her daughter, and somehow - no evidence how - contacted her brother to help, and no one saw his car, and the boys slept through everything, and he's flunked a polygraph too?
I mean, please help out here with what *could* have happened, realistically, in terms of body disposal of this child - if she did NOT drive off with an abductor. I am genuinely interested in people saying what they think could actually have happened. Without a magic carpet or ESP to the brother. Is there a fourth cell phone on which she called him?
Because I'm just not seeing a scenario in which the dogs wouldn't bark, the kids wouldn't wake up (and tell the cops about it), and the neighbors wouldn't be alerted to a car pulling in and out in the middle of the night.