There was a really great post on Skepchick about grief and loss that I wanted to link to, and share my comment:
I spoke to a man I know at work after the death of my grandfather. I was having a very hard time coping with the fact that I didn't feel the loss as much as the rest of my family, and that when I did think of the loss, I felt more like it was unfortunate, but I didn't feel that sore feeling in my heart that I still feel when I think of the loss of my great grandmother (who I didn't know as well, but I think I felt more love for) and my great grandfather.
He said, "The only reason to feel sad after someone dies is because we don't want to stop having them in our lives. It's selfish." He explained that whether you believe in heaven or not, the only reason to not want them to die is because we want them to be here with us, not for any worry of their suffering or lack of existence.
Since then, I've had a very different outlook on death. I still feel sad when people die, but I acknowledge WHY I feel that way. I haven't lost anyone very very close to me, but I think about it a lot.
The way I see it, regardless of whether I believe in heaven, the last few seconds of brain activity can feel like an eternity, and many people see the things that make them the most happy in that moment, and that can be their heaven, and I would never begrudge anyone that momentary happiness. I try to think of that when I think of how sad and scary death can be.
Grief is hard, but it is a selfish thing. I think remembering the lives of the people we lose, and doing things in their honor, instead of feeling bad that we don't have them anymore, is the best thing we can do. It is amazing what you're doing to honor your sister!