In-Laws

The logic behind the names for relatives gained through marriage just does not make sense.  Let me first start by pointing out the obvious. Dating and/or marrying your sister is wrong and disgusting (as I'm sure most of you will agree).  Now, let's look at a man who marries a woman with one brother and the standard [aside: this term is questionable] set of two parents.  Once the man and woman are married, the woman's brother becomes the man's brother-in-law.  This is fact. He is essentially your brother, only by legal documentation.  So then, his sister would also be your sister, even if only by law.  Thus, I can only conclude that this man's wife is also his sister-in-law.  The man has married his sister, which we all agreed was wrong.  The same conclusion is made if one follows the logic of one of the parents-in-law.  Why would the term "sister" be chosen for a man's wife?  From here, you can believe one of two things.  Either sibling marriage is okay or the names for in-laws need to be changed.  I personally prefer the second option.  I'm not a language expert, so I can't use the origin of "brother" and "sister" to offer you better terms, but someone needs to.  If it is too difficult to create a new term, at least bump them back to cousins-in-law.  While still wrong, the weirdness of the terms is less obvious.