@glitterbag,
glitter, I can't find anything saying step brothers and step sisters can't marry in the usa, whether they were brought up in the same household or not.
Could you please site where you got that information?
In any case, even if it is illegal in some states for 2 people who share no blood to marry, they could easily cross the state line and marry there.
If the thought is that it's illegal or wrong because they were brought up in the same house, wouldn't the same apply to non blood related people who grew up in any number of common/communal living situations? What about foster children from a wide array of parents, who are raised from early childhood to adulthood in the same home?
I can't get my head around what would be wrong if there is no blood relation, and both parties enter the relationship willingly, as adults.