sparky, this exact same thing happened to me. My mother took care of my older daughter (my daughter slept overnight and spent a day with her every week from the age of 4 months on) and she died when my daughter was 3, almost 4 and my youngest was about 6 months. My children are now 14 and 11.
The most important thing to remember is that children this young do not have the same thought process as adults. They are also not able to verbalize their feelings very well, if at all. The truth is, you daughter will probably not remember much about your mother, but that is ok.
First thing I would do is monitor her behavior. Because they are not so good verbally, sometimes they express their sadness in other ways (bed wetting in a toilet trained child, abandoning toys). Try to talk to her about these things if they occur. I would not mention anything about your mother dying or where your mother has gone unless she does ask.
I would find "teachable" moments to talk about your mother. If you are decorating a tree for Christmas and you find an ornament she gave your daughter, talk to her about it. Look through photo albums. Both my kids loved watching my wedding video, where they could see my mother laughing and dancing. Encourage your father to do this when he is ready. Answer all her questions honestly (don't say she "went to sleep" or "went away").
To be honest, I don't really think my older daughter actually remembers my mother, but she is able to talk about specific time she had with my mother, I think mostly because we talked about it so much. My youngest did not know her at all, but still asks about her and calls her "My Nana".
I'm so sorry for your loss. This is such a difficult time. Please know that you are in the thoughts of many.