I completey understand where you are coming from. My mom passed away 11 yrs ago so when I got pregnant I wanted to name my child after her. I also wanted to name my child after my grandma that had passed away. While I was 5 months pregnant my mother-in-law was diagnosed with Leukemia and we knew she wouldn't be around for long. So, I had 3 people to use their names, but knew it would be too painful to actually call my child that name so here is what we did:
Mom: Carole Grandma: RoseLee Mother-In-Law: Judith/Judy
Daughter: Carlee Juliet
We used the letters of all their names, so her name has so much meaning to me and yet when I call out to her, it is not awkward or painful and she has her own identity.
I like the idea of using Ana as Karen had mentioned - either as a first or middle name. You could also change the spelling Suzie, Suzana. Or create a whole new name with parts of your sister's name: Alana.
You can also look up the meaning of your sister's name and find another name that matches the meaning.
For a boy you could use something like Zane - if you wanted to carry on with your sister's name or you could stick with what you already like.
Good luck to you - sorry about the loss of your sister - but congrats on baby #3!