In the beginning, God created Adam and Eve. They had children but now what? There were no other humans on the planet so the children had to have sex with each other in order to have more offspring. After that, their children and grandchildren also had to have sex with each other and on and on.
Eventually God wiped out the Earth with a flood.
The only Humans saved were Noah, his wife and family. Once again, no other humans.
They had to have sex with each other for multiple generations. Technically, we are all related therefore any sexual intercourse is incest.
Apparently God doesn’t have a problem with that. So go do your sister. It’s OK.