It is odd to me as well.
People in shock are not as articulate as she is. They may not be flailing about, nor crying, shaking or screaming, but they tend to be confused and in a state of unreality:
Acute stress reaction - Wikipedia, the free encyclopedia
I went into shock as a child due to a trauma. No way would someone in psychological shock act the way this mom did. She actually snickered when Jane pointed out how strong she was being - grinned, laughed and flipped her hair.
I am also very familiar with tranquilizers. She has none of the signs of someone who is on one - at least none of the signs of someone who has taken enough to be calm in the face of great trauma or fear.
That being said, I think it is very clear that this family are victims. Mom of Lyric was at work. Dad apparently was as well.
Both kids were with grandma when they went missing. To believe that mom's reactions show consciousness of guilt would be to believe that dad, mom, grandma and the parents of the other child, are all in on it, as well as possibly the aunt - that either all of them conspired to harm the girls or all of them are conspiring to cover up what one of them did.
That makes no sense.
I find the mom's reactions very odd but maybe she is simply telling herself that they are okay and will be coming back. Maybe she is in hardcore denial.
Bottom line though, the facts indicate there is no way they are involved. They are victims.