-Thought/memory/flashback - Connect to what's happening now - Then straight to action
So I'd probably start with the MC thinking/remembering a special memory with her sister, maybe something along the lines of:
"Hide and seek. My sister loved to play that game. Me? I hated it. I remember the first time I was the one who counted and she hid. For five minutes it was all fun and games as I tried to figure out were she was, then I started to panic. She was gone! I started crying and screaming for her, I could not imagine life without her, how could she be gone? Eventually, she came out of her hiding place and consoled me and we had pancakes together, but I will never forget that panicky feeling. When my mother called me that Friday morning, saying she had not heard from my sister since last after noon, my increased heart rate and sweaty palms gave away that I was starting to get that panicky feeling again. Was my sister lost? Could she really be gone?"
By introducing the different characters at time of grief, you can also show what they are like through action. Like "She did not know how to console her friend. Her hand hung awkwardly in the air, as she did not know whether to pat her friend's shoulder or hair. She hated that she could not do anything to help." instead of "she was socially awkward".
As for the murder, it really depends on what you want the story to be like. I kind of liked Calla's idea that the sister might still be alive somewhere, sort of like The Girl with the Dragon Tattoo by Stieg Larsson, if you've read that book?