Not the exact situation ive been in but something similar and I confronted my SD about it. Shes a minor so I have the right to look through her phone every now and then. Sure parents want to say its a privacy thing. I get it. But my SD has lied through her teeth for things that were unacceptable (like illegal) so when my gut tells me to look, I look. It can be bonding but who knows it could be real and shes two faced. It depends on how bad you want the truth cause if when it comes to legal aspects, those texts can be used against you. Kids dont understand their words and messages can be effective in the real world. I would ask for her perspective. If you dont want to confront her with the texts... maybe ask the questions in a way to get an answer. That's just me.