You get through it day by day, moment by moment. You find a joyful moment and you hold it tight; rainbows will always hold more meaning in my heart now, even tho' by birth, rainbows are a part of my Irish heritage. You take it a step at a time and don't rush; the pain will soften but Kristina will forever be in your heart.
You're right that nothing makes sense. I don't know your beliefs but I can tell you how I feel now. It doesn't make sense that God would give us a wonderful, beautiful child, then something happens and we end up having to give that child back to God. God made earth and everything on it to grow in it's own way. God isn't a micro-manager. You may find some comfort there in time.
I don't know how old Kristina's sister is, but find the book "What's Heaven" by Maria Shriver. My son found it very helpful. He misses his sister too and he always will.
You are so welcome. Please post; we're here to help.
Many hugs,
Joe