Dear Cat, I can't even begin to tell you how sorry I am for your loss. Losing a sibling, even a step-sibling... That's really tough. And my heart is breaking for the children, both hers and yours, as well as others in the family. When they're so young, they just don't understand, do they?
You and your family are in my prayers. 
By any chance, is there a particular color or gemstone or animal or favorite object, or anything like that, that you think of when you think of your step-sis? I'd like to send you a little something in remembrance of her. Cat, I can't for the life of me remember what faith you practice. You're not Catholic, by any chance, are you? Yes, the logical continuation of that thought process is that I'd love to make a rosary or rosary bracelet for you to remember her by, IF you're Catholic, that is. If not, I have other ideas...
Cat, for the children's sake especially, but also for you and the rest of your family, is there a chance there could be a memorial service of some sort held? Don't forget how important a ceremony recognizing your step-sis's passing, and celebrating her life, is in the grieving process. When my mother died, it wasn't until the interment that I finally accepted she was really gone. As difficult as the wake, funeral, and interment were, they were important in helping me to grieve in a healthy manner. I guess I'm mostly thinking of those precious little angels when i worry about there not being any memorial service for your step-sis. It doesn't have to be elaborate, either. It could be a gathering in someone's home, offering those who wish to speak an opportunity to share memories of your step-sis' life, as well as prayers.
I love the idea of memory albums. I wish I had done something like that to remember my mother by. None of my children ever knew her, she died when I was pregnant with my first. I've talked about her over the years, but an album with photos would have made her more real to them, I think.
Cat, you're strong girl, hang in there. Don't ever forget that He never gives us more than He knows we can handle. (Sometimes I wonder, though, if He's confused me with someone else! ;^) ) And remember how relaxing and therapeutic the beading process is. Why don't you bead a cover for a journal to record your thoughts? Or just sit down with a pile of mixed beads that need sorting. You're in my prayers, friend. 