I would only tell her if she, or you could wait until she was about 10. I think at this age its too confusing/ hurtful to try to understand why a parent would not want to be with them and would not do the things a parent is supposed to do. When they are older, they can gain some more perspective, and digest this information easier. Not that it won't hurt then, but with age comes wisdom and tolerance.
First of all, does she know that her stepfather is not her biological father?
Secondly, is she asking questions about who she is or where she came from? (Such as, if she knows her stepfather is not her bio father, is she asking about her bio father? Or if she doesn't know her stepfather is not her bio father, is she asking questions about why she looks or feels "different" in some way, for example, the last name difference?)