in my country it's a little complicated, but if you can understand it it's really easy,
marriage is allowed between family member of the 4th branch of relation where the likelihood of blood relation is most likely 5% and not more, it's pretty distant relation to be honest, and correlation of blood and DNA is normal at this level as most humans share around 3-5% DNA information,
1st branch:
parents, gramps, siblings, in short, direct relatives.
2nd branch:
parents(1st) siblings and their children
3rd branch:
parents(1st) siblings partners(2nd) (wives/husbands) siblings and their children
4th branch:
parents(1st) siblings partners(2nd) siblings partners(3rd) siblings and their children
quite far as you can see, in a direct interest line of curse so it's assumed to be safe.
Never heard of any 3rd or lower branches marriage attempts, other than royalty of curse
