Knowing kids the way I do, I'm wondering if it was planned at all. My first thought was that the older girl was going to do something she wasn't supposed to or just to get away from people for a while (more likely if she was being treated for anxiety or depression), and Katie wanted to go with her. I know I drove my sister nuts, my younger son did the same with his older brother and many other kids I've known did the same by following along where we weren't wanted. I know I got hit many times for not going home and giving my sister some privacy, although nothing life threatening ever happened. Even if the girls weren't sisters, young kids do often idolize older kids and sometimes act like unwanted shadows.
The other thing I wanted to mention is something that's really not my business, but I thought I'd post a reminder anyway since there are some very new posters. Not only are we supposed to make sure we're posting about the correct person (thinking of the parents and the couple who lost the son in 2005), but we're also not supposed to sleuth family or friends of the victims or suspects (or use the full names of other children, if I remember correctly). I don't think anything posted so far has really crossed the line too far, but it might be a good idea to keep it in mind.
MOO