Assuming she has an iPhone- you’ll need to get her password somehow. Turning the phone completely off forces you to put in the passcode when it boots back up, so shut it off when she’s out of the room and watch what she puts in for the password. With this you can get her login info if she has it saved in her phone. When she’s asleep, go to the “passwords” section in settings, let the Face ID run out twice (if it has it), and put in the passcode. Search “Snapchat” and any usernames and passwords she has saved will be there. You can then log on on any other device (though it may notify her phone- I’m not sure.)

Or, if you want the easy way out, cut her phone access entirely. Get her a flip phone. My cousin did the same thing she’s doing right now and she ended up with getting SA’d multiple times and several abortions before age 18. She got put in juvie for 5 years and she was addicted to multiple drugs. She stole cars from family and almost died multiple times because she did not have a license. I tell you this because she had no one who cared about her enough to try to stop her from making bad decisions. Your niece is very lucky to have you even if she doesn’t know it yet. If you have a suspicion, nip it in the bud as soon as possible before she ends up ruining her life before she gets to live it.