From my perspective, there are going to be jerks everywhere who poke and prod at the sensitive types because they get a rise from drama. It doesn't matter to them if their point in nothing but a load of bullshit, all that matters to them is the attention they can get from stirring up the pot.
Fortunately, from my time here I have seen very little of that, which is the main reason I considered letting her join. I can't shield her from every asshole out there and I'd end up going insane if I tried. At least here I can monitor what gets said to a degree and step in to my mama bear role if need be, without it looking too bad on her part. She's not an instigator like her father can be, but she definitely isn't afraid to keep things going and she hasn't quite grasped the concept of not feeding the trolls yet.
My major concern in all honesty is the attention seekers who make outrageous claims of a horrible life to manipulate the more naive individuals that buy into that stuff. That's everywhere as well, but again, I haven't seen too much of it on here.
Your basically describing troll's and attention seeker's; which has a lot of overlap demographically.
The honest truth about Iwaku, is that the core Iwaku community (the most active and/or senior member's and the staff) are the ones who set the tone for the whole community in general; and they are all caring, compassionate, (mostly) mature people. Sure some of them have pretty sarcastic, insane, and dark sense of humour; but at the core they care. And they are the people who will be looking out for her and other members who are vulnerable like her; and believe me they will protect her. The mods on this site are very protective of those most vulnerable in the community (and the community in general), and with all different times of people with many different issues; they have to be. I think honestly Iwaku is one of the safest places on the Internet, while also being a tonne of fun.
Your worried mainly about someone who is making "outrageous claims of a horrible life to manipulate the more naive" well for one you could warn her of this possibility when your on the Internet, but also when she's on here you and the mods can deal with that possibility.
Sure there is the possibility of something going wrong, but that's always the case; and the possibility is reduced when it's on Iwaku; especially when your here to look out for her. You don't seem worried by the idea of her invading your safe haven or being to close together for comfort so I'll defer to your wisdom on that. I think that if she want's too she will find a place to RP or an online community, she will find one and it could be a much scarier place. Unless you monitor all her Internet (which would be very invasive and most likely very bad for your relationship), and this way is much better so you can monitor, without her feeling your so invasive and having a good reason to do it (you being a community volunteer and a member).
I advise let her on. But you could always wait until she's older, more mature, and/or in a better frame of mind. Or never at all. Ultimately its your call, but honestly I think you want to let her on your just having doubts; which is completely understandable, but I think you unconscious may already know your answer one way or another.
