In Queensland, Australia, incest laws prohibit relationships between step siblings.
Divorced man & woman marry. Their non-local kids also marry each other. Is this legally problematic?
But oddly enough the statement of marriage rules here doesn’t mention that: it just says that you can’t get married if you are “in a relationship” with a sibling!
Kind of odd. If one didn’t know better, one might assume from that, that being “in a relationship with a sibling” was no more illegal than being in a same-sex relationship.