I have no idea why this was posted in the Poetry section, but we don't even need a biblical mandate for this. One, in the US it is illegal, at least to marry. Two, genetically speaking, you have a high probability of your children being born deformed. Research incest birth deformities.
And, if you still insist on a biblical concept, there is scripture against incest
"No one is to approach any close relative to have sexual relations. I am the LORD," Lev. 18:6