Hi. Nerve wracking stuff! Tumblr is literally just a sight where you post any content (pics, journal entries). It can be about that which I've never seen. I've only seen it vein used to post vacation or food photos. Things like that.
I figure at this point, unless you take the computer completely away it will be almost impossible to track what our kid sees. My 8 year old has accidentally seen a few very undesirable things online. Thank heavens he always tells me and asks me what it was so I have a chance to put out any possible forest fires in te future, of you know what I'm saying.
Another thought, 16 year olds are so moody. I remember when I was exactly 16 I went through a phase where I was obsessed with The Cure. I was in a very dark place because of it, writing some seriously dark poetry. My parents questioned it but never took it away from me. Did they steer me through it, yes! But I eventually lost interest and moved onto brighter, sunnier things. Teenagers are so fickle. Hopefully your niece is going through a phase too. And maybe if she has no guidance, you can be that for her? Hope it all works out.