Page 8 of the translated chapter 1 mistakenly says "those who are blood-related within 3 generations are not allowed to get married". This would imply that you can't marry siblings, first cousins, second cousins, first cousins once/twice removed, uncles/aunts, etc.
But the actual text of the law says that the prohibition applies to "relatives within 3rd degree of kinship".
(Assuming blood relation)
1st degree: Parents and child
2nd degree: Siblings, grandparents, grandchildren
3rd degree: Great grandparents, great grandchildren, uncles and aunts, nieces and nephews by your siblings
So cousins are legal.
The fun thing with this law is that you can marry your sibling's grandchild. Yay!