Step siblings and relationships Lord Buddha once said 

"The future humans will have unusual lusts and temptations"

So when i saw a this in a TV series i couldn't define the line,

If two step siblings form a romantic relationship is it against the five precepts?

So what if we say they are independent individuals,What then?
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.