Genesis 5:4
And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters
It's apparant that Adam had more children than just Abel, Cain, and Seth. Adam lived 930 years! In theory, he could have had up to 400 children of varying ages (Seth at 130, one child every other year afterward over 800 years is 400). And incest wasn't expressly forbidden, as far as I know, until the Law of Moses, more specifically,
Leviticus 18. Before that, I can think of nothing that forbade brother-sister relations. After all, Sarai was Abram's half-sister.
And as for horoscopes, they are wrong because you are trusting your future to mere
things, the sun, moon and stars. At some point in history someone thought that this group of stars looked like a bear, etc. That's your history of constellations in a nutshell. Anyway, it is wrong to place our trust in things. The thing has no power to either predist or cause some future event. Only God knows. To trust a horoscope- or any fortune-telling at all- is to say that you do not trust God's providence.