Bellion's Favorite

I found out yesterday that my 13 yo niece watches anime/amv/yaoi videos on her tablet and I don't know how to feel about it. I noticed that she's always hiding what she watches and acts suspicious whenever I ask her about it so I secretly checked the history and found the videos. Some of them are actually 18+ and contain ecchi fanservice, boobs, male characters kissing and probably more... I immediately changed the censoring option to hopefully hide some 18+ content and I think she doesn't know how to change the settings yet. I tried accessing her gmail account from my laptop to make more changes but wasn't able to because I don't have the password. My siblings and I were against my niece having a tablet from the start but my sis/her mother didn't care and just wanted to keep her busy. I'm pretty sure she thinks her daughter only watches cartoons and plays games and my niece totally plays her mother and lies to her. I can't really intervene because I'm the youngest in my family and rarely taken seriously and to be fair, I started watching/reading 18+ content at that age too but we didn't have the internet at the time so it was just TV and books till I was like 16 and I was still using the family pc so that limited my exploration but my niece has access to the internet all the time and on here own private device and I feel like it's way too early for her age. I mean, I was pretty shocked when I first read my first yaoi and I was 20 yo!!!!
What do you guys think I should do? I only have 2 options really: try to forget about it and treat her normally or secretly intervene, take the password and monitor what she watched from my own device.