Thank you for your question. The best way I can answer your question is to explain that Cain & Able would have married their sisters and thus began the population of the earth. This is assumption, since women were typically not included in geneological listings - only the men.
This was possible for two reasons:
1) Genetic material was absolutely pure - prior to the destruction
of the water canopy that once protected the world from the more damaging
rays of the sun, people lived long periods of time. However, since that
time, genetic makeup has steadily deteriorated. Today, when siblings marry,
it results in deformed or handicapped offspring. But, in the beginning,
this was not so.
2). God did not declare incest to be sinful until the Law of Moses. At that time, it became wrong (presumably, in part, due to the genetic issue). Prior to the Law, it was not forbidden for marriage between siblings, cousins, etc.....
Hopefully this will help you and your daughter explain the origin of humanity.