It sounds like a tough situation. I've been finding that answering the questions with only as much detail as necessary is enough, as some others have posted. It can get tricky when there is a little bit of contact, but not much and without any consistancy. So you are lucky in that your daughter will not have to deal with the sometimes here, then gone for a while scenarios that can cause extra heartache.
I wonder though, why you feel that your daughter has no concept of a mother. You may be surprised how much she understands. If she reads books (or has them read to her) and has some friends at school, then she has been exposed to the concept of Mother. Also, if you've ever mentioned your own Mother, especially with your brothers then she has an awareness of what that means. However, she most likely doesn't feel bad that she is lacking one; she just doesn't have one in her life... if that makes sense.
An example is that my almost 3 year old hasn't seen her dad since her 2nd birthday party. She has seen him a total of 4 times since I left him when she was 4 months old. They did talk on the phone off and on over the past couple of years, but not very often. She recently said "I want daddy, I want daddy". I had to explain to her that he lives far away, and it is difficult for them to talk. He lives in England, where he was born and raised, and he finally went back there this year (so his harrassment has subsided significantly!) Anyway, I told her that he loves her very much, but that we couldn't all be together. I asked her if she was sad about it, and she said no. We have had the same conversation a few times, and each time she always says that it doesn't make her sad. By the way, she is very aware of feelings, and likes to point out how people (or characters in books) are feeling by their facial expressions or their comments. So I know that she knows what 'sad' means.
I'd continue what you are doing, providing a loving family for her, and answering her questions about her Mother as they come up. By keeping it positive and only telling her the bare minimum, it will let her know that there isn't anything wrong with her family, it is just the way it is.