Arkguy:
Are you saying that a straightforward, plain reading of the account doesn't seem to indicated that there were others living at the time of the murder?
The simple chronology of the literal story is very clear. Adam and Eve have the brothers, the brothers have a rift and Cain kills Abel. Cain is condemned and is worried that when we wanders, others will kill him. There has not yet been any indication of anyone else being born to Adam and Eve. Cain heads out.
Since Seth was born when Adam was 135, and the text STRONGLY implies that he is the first born after Abel's death, then this all makes for a VERY strained reading to imply that Cain married his sisters. If you want to give a model that works without getting very far from the plain reading of the text, go right ahead. No matter what, you would have to read in an AWFUL lot into the text, and read it in a very odd way to get the result you want. The bottom line is that NOBODY would ever read that into the text unless it was absolutely imperative in order to fit with their other beliefs.
And the genealogy given later does not contradict the chronology concept, as you suggest before, since when someone is giving genealogy, they will very naturally start again at the first person in the lineage.
Now, as for evidence of pre-existing humans, the evidence is all inferred rather than explicit, much like your theory about Cain marrying his sister. Both theories require reading between the lines. The only thing is that the Cain marrying his sister idea takes a lot more gymnastics to arrive at its point.