As a mother that had to go thru this exact same thing when my daughter passed away..... might I suggest that you take it day by day and that you answer her as she has questions and always keep a few pictures of her brother still out in the house - for her to remember him by and for her (and the family) to continue to heal.
Bottom Line - it takes time.... it took 4 years for my family to learn how to live again.
LoVe,
Rhapsody -