Probably some of both. That's usually how things work when you're a kid. The other kids were her sibs or kids from her school I believe according to anything I've seen on the news about it. I don't think its necessarily uncommon for one child to be scapegoated or picked on to some extent by the other sibs, especially when you have a situation where two can pair off against one - the older brother seems kind of out of that little triad. Different age, etc. You do kind of wonder why she'd be picked on as much though given that she had a twin. I thought twins were kind of more a pair thing (of course, I don't have twins, so I'm prob just oversimplifying).
On a separate note, I do think this is going to be an especially difficult loss for her twin brother, now and over the years to come.
I don't want to go into victimology with this situation too much because it causes a lot of strife here and won't necessarily be productive, but I am of the belief that this child was a little more at risk than some others her age might have been. There are reports of her being overly friendly, never met a stranger, that sort of thing. Of course, when you are talking about a seven year old, all of that becomes relative once you have an offender who has decided to target that child. Even in the most optimal situation, a seven year old is no match for an adult predator. But, predators do target the children they can offend against the most easily.