oh wow i'm so sorry for your loss. i sort of know how you feel, except thhrough different situations, and it hurts like hell, just the concept of never seeing them again and especially if they're young (my older sister died when she was 9 and my cousin when she was 11, and it's just the what type of people wouldthey have grown up to be, and it's hard when i feel like i really need the support of an older sister). but it will get easier in the future, you'll always miss them, but i believe that they're always watching over you and will be there in spirit when you need them.
in terms of not being able to practice, i find practicing piano a really good way just to vent my really strong emotions when i need to, so see it as an emotional release, even if you end up bawling your eyes out in the middle of a piece, but it does help, especially if you can associate something you're playing with your friend, and then play it like a tribute to them, and she'll always be in your heart