How can I help my 6 yr old niece understand death?

I desperately need advice because I feel like my 6 yr old niece will be permanently damaged. About a 1 1/2 ago my sister died suddenly while visiting our mom out of state. It was my sisters wish to be cremated so my niece was unable to have closure by saying goodbye. I did my best to try to explain to her that her mommy got very sick and died, and now she is an angel with God. My niece does not understand and still believes that her mom is visiting grandma and just hasn't come back. When she comes to stay with me on the wkends she sometimes asks where her mom is (my sister lived with me). I continue to explain to her that her mom is in heaven and is unable to come back. The second part of this tragedy is when my niece's went to live with her dad full time after her mom died, she latched on to his wife and they became fast friends and basically adored each other. Of course this was very hard for me to take, and we had a very rocky start because I was worried that she was trying to take her moms place, but once I gave her a chance I came to realize that she was just an incredible woman. Instead of trying to take her moms place she did her best to help her to remember her mom. My niece began to think of her as her mom, and strange as it may sound, she became my best friend. A few months ago her stepmom found out that she had cancer and got very sick-very fast. She went through her 1st round of chemo 2 wks ago, and she died this past Wed. So now my 6 yr old niece has lost 2 mommies in under 2 yrs. The worst pt is she is the 1 that found her that morning. She was able to go through the entire funeral process. It is pretty obvious that she has retreated mentally. She believes that she is asleep and will wake up. We have explained to her that her mom's (#2) body was just too sick to keep going. She goes in & out of moments where she just looks lost, she won't eat or talk, and just stares off. When I try to talk to her about what death means she just walks off. I'm really at a loss. I know a counselor is a must, but does anyone have any ideas of what I could do in the mean time?