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I also agree the sister was tempering her reaction for the benefit of the baby. I have a child that age. If, God forbid, I would walk in and find my husband in such a way and my kids were there, I would hope I would do my best to stay calm and not start screaming and go into hysterics because it would only cause my children to do the same. She was trying to keep the baby out of the room the mama was in, I would have done the same. Remember that poor tiny girl didn't have her primary support system, and Meredith didn't know whether the girl saw anything, how long her sister was dead etc. When I first listened to the 911 tape I was disturbed and suspicious, but as I carefully weighed everything, I find Meredith's actions reasonable.
LE is no doubt going to keep this one close to it's vest in order to NAIL everyone who had any role in this. ETA they did talk about the vehicle seizure, probable cause versus order of testimony in the above article (Sabal's post)