Uhm... I think Rena has always known Shion actually, because Rena was from Hinamizawa all along and just moved away and back later on. Probably knew her from before, I'm not really sure thought.

But Satoko, for example, and me... In some worlds we never saw Shion. When I met her I even thought it was just Mion showing a side that she didn't show often. And in those worlds normally she ended killing us all... Mion or Shion.

I'm not sure who did it, but knowing that Shion's state was quite serious maybe I was wrong back then thinking it was Mion the one who killed us.