"Step-siblings can get married."
... what?
It's not really unique to Japan either, you can do that here in Germany too. Or in the USA for that matter, seems p common overall.
I guess step-siblings don't have to worry too much about biological incest, so its legality is less of a concern.
To be fair, the biological problems of incest are only a small issue. First you do not have to get pregnant if you have sex (protection does exist) and if we consider the possibility of bad genetics a problem then we have to ban sex between people who already have inheritable genetic diseases.
The big issue with incest is the power imbalance. Older sibling/younger sibling father/daughter aunt/young nephew etc.