What if the girls stumbled upon a drug... a drug that one of the girls might be familiar with as in had seen a family member do this drug. That child then trying to be the older, cooler cousin shows the other girl how to do the drug. The second child follows suit and takes the drug as her cousin shows her. They overdose and are found by a family member who panics and attempts to cover it up.
Believable to me. But then the person who found them would have to have known they took the drug or be able to recognize the drug on/near them too. The thing is, if I found one of my children dead from drugs ( be it Rx or illegal) I am calling EMS not hiding bodies.
I noticed on the early videos of the family interviews that most seem calm (only one seems very nervous). IMO