It can be hard to find a balance between being overwhelmed and pushing it all away. Someone else already said we each greive in our own way. There isn't a "right" way to do it.

I found what worked for me was when it got to be more than I could deal with I'd push it away for awhile by distracting myself. Then I'd let it come back again when I felt ready.

HAving trouble believing your sister is really gone is not uncommon.