i assume this question is 'Why cant a brother marry his sister, if they are both alone and do not not like who they are married with?' <br />
Well, the answer is pretty obvious because it has to do with ******...if you do not like who you are married with,leave...but even though its none of my business I highly recommend you do not marry your sister...
Why even bother getting married?. Just live and sleep in the same bed with each other. Then you can regulary have sex and some babies when you both want to.