Oh mama. I am *so* very sorry for your loss.
My son died when my dd was 3 (she is now 6) and she still talks about him to this day. She grieved just as deeply as dh and I did (that was very surprising to me), and she still grieves to this day. We are Christians, so we just explained that Ryan went to live w/ Jesus. She accepted that, and would always ask what he was doing up there, etc.(we always said he was swinging, etc).
I tell you that to let you know your dd will grieve, and probably deeper than you would imagine. It's ok. Talk about your dh w/ her-keep his memory alive by talking about the good things he did, the fun times, etc. We put pictures out everywhere of Ryan, we talked about him all the time, etc. I made sure to tell dd that we could talk about him anytime-and to not feel bad if it made mommy cry. I think kids bottle it up sometimes b/c they don't want to upset their moms/dads, and obviously that's not good. My dd went through cycles in her grief, just as I did. Some days she cried a lot, other days she was mad, etc.
You could check w/ your local Hospice for a list of appropriate books. They were very helpful w/ us when Ryan died-and he was never associated w/ Hospice.
Please know you and your girls are in my thoughts and prayers.