There is something wrong with the coding in your theme. The first post is coded correctly, except the next post starts before the end-of-DIV on the first post, and the third post starts before the end-of-div on your second post, and the sidebar is contained within the code for your last post, since it starts before the end-of-DIV for your last post.
There is at least one </div> tag in your theme that is in the wrong place. I ran your homepage through the page validator
and there are 73 errors. Some of them are these div's. If you work on cleaning up the errors shown on the validator page you will eventually find the error source. And that will solve your sidebar problem also.