My girlfriend's(23f) sister(11f) died in a car accident last night. What do I do?

I've never felt loss like this. I can't stop crying. She was such a sweet girl that I helped raise and I feel like a piece of me is gone and I'm trying so, so hard to be strong for my girlfriend but I don't know what to do.
My girlfriend watched her and raised her, but she's dealt with death her entire life and she seems to be handling it. I don't know. Maybe im the one who needs help. I just don't know. I feel so discombobulated, and lost.
I'm sorry if this is the wrong sub I just need to know what to do.
Tl;dr need help to be strong for my SO.


Source.