Back to the point at hand... How the heck can I keep an eye on her (one computer in the house, by the way) to make sure she's not getting fed bad craziness?
Gonna try to keep her away from her cousin, but that's gonna be hard since the whole damn family is VERY close...
1. Computer is in open area (living room, kitchen, whatever)
2. Bite the bullet and go buy:
1) New LCD monitor
2) Wall-mount for LCD monitor
3) Powered video splitter cable to get the video signal to existing and new LCD monitor
3. Mount the LCD monitor above the computer table so all can see
4. Some sort of net nanny software is fine, but make sure you also get a software firewall that you can configure to log all incoming packets & the sites they come from. Spend 15 min/day seeing just where the little gal is going. Block objectionable sites.
5. Administrator access for ADULTS only. Kiddos get user accounts. Disregard whining when they can't install every bit of crap/mal/spy-ware disguised as video games.
Kids have no business accessing a networked PC by themselves. In your case, it seems your GF's daughter is looking for trouble. Well, sometime trouble looks for them, too.
As to the cousin, I would do my best to keep them from hanging out together. Your GF has to be on the same page of music.