Step siblings and relationships
As per the 20 unsuitable women listed, it could break the precept, depending on if one or both are in care of parents/relatives/guardian or if the king/government prohibits such relationships or if it's prohibited by tradition. 

But one should keep in mind that taboo &amp; obviously vulgar behaviors are not discussed under the 3rd precept. That doesn't mean that they don't fall under sexual misconduct.