God did. In Leviticus:
None of you shall approach any one of his close relatives to uncover nakedness. I am the Lord. You shall not uncover the nakedness of your father, which is the nakedness of your mother; she is your mother, you shall not uncover her nakedness. You shall not uncover the nakedness of your father's wife; it is your father's nakedness. You shall not uncover the nakedness of your sister, your father's daughter or your mother's daughter, whether brought up in the family or in another home. You shall not uncover the nakedness of your son's daughter or of your daughter's daughter, for their nakedness is your own nakedness."
Leviticus 18:6-18
And perhaps reading this would help:
Why did God allow incest in the Bible?
Thanks for that. I didnt know Christians gave the same answer as Jews.
Say what now?
The answer you link to is the same answer Jews give. Equally with the question of how Yitzchok could marry two sisters when the Torah specifically forbids it. Although that answer is more complicated.