Is God Prohibiting Close Marriages?

The prohibition on close marriages comes from the time of moses with the mosaic law. Adam and Eve had more children than are mentioned in Genesis five, verse four. So if you put all those pieces of the puzzle together, it seems obvious to me that there were sibling marriages in those early generations. But i don't think it was so early in the history of humanity. And even abraham marries his half sii, was going to mention that abraham and sarah are siblins,. yet they are sibling, so they're half siblings.