It's a strange construction, isn't it? I can see why it puzzles you. The only explanation I can offer is that we sometimes use the verb 'saw' to mean 'This is what happened'. For example, you might come across a sentence such as:
"2008 saw a dramatic fall in share prices".
meaning
"In 2008, there was a dramatic fall in share prices"
So the sentence in this article can be read as
'In one incident, three sisters were attacked ..etc'
It's an odd kind of sentence, but it isn't uncommon, especially in journalism.