Sigh.
You people do realize that a non-blood-related sibling relationship is legal in parts of the civilized world, right?  
I'll use the US as an example.  There are 4 different kinds of sanguimony laws between the states.
1.  It is legal to marry either a first cousin or a step-sibling (which Sachi legally counts as)
2.  It is legal to marry a first cousin but NOT  a step-sibling
3.  It is legal to marry a step-sibling but NOT a first cousin
4.  It is not legal to marry either.
Now, I'm guessing you would expect the first option to be the most common.  It isn't; the middle two options are by FAR the more prevalent ones.  The distribution is something like 5, 20, 20, 5 (from 1 to 4).  [Yes, most of the states where you can marry a cousin are in the former Confederacy.]
Of course, those laws have nothing to do with what is considered socially acceptable.  It is also worth noting that former step-parents are included with step-siblings.