Ok dude, you're being overprotective.
Your sister has a different view than you. Women mature a little faster than guys most often.
It's great that you're religious. And you're sister hasn't lost her faith, she's just discovering who she is. Ratting out your sister or bugging her phone will only drive her away. And you'll loose her faster.
Take the advice. There are ways of positively influencing her decisions that don't involve parents or discovering porn on the computer or phone.
And some advice, if the parents discover it on their own, take the hit on her behalf, no matter the punishment. Because you sister will trust you more. She's obviously not talking to your folks, you want her to talk to you at least. If you cover for her porn habits, you can let her know she can trust you. Which means when she gets older she voluntarily come to you first.
As for influencing her decisions, reaffirm good choices, don't judge the bad ones. If she starts to date a boy who is a jerk, tell her : you can do better.
If you see her dressing like a tart, remind her that she's beautiful in another outfit she owns that isn't as tarty.
But don't judge. She's your sister, you're not her dad.
She's going to need a friend more than often. Let that person be you.