This might sound a bit... cold, but I don't think Tylee cared that much about Charles. It seems like she was down to ride with her mom and her uncle, and that she actually had a somewhat "bad" relationship with Charles. I think harming JJ would be crossing the line for two people: Tylee and Colby. But Tylee would be the hardest one to keep quiet because she was with JJ everyday, it would be really hard to come up with something believable to explain why JJ was not around anymore. Colby, on the other hand, even though it's obvious that he was present in JJ's life, had his own family and his own life, and was not in Idaho so lying to him about JJ would be easier. I believe Lori kept Tylee's phone to communicate with Colby pretending to be Tylee so he wouldn't get too suspicious.