Twenty eleven theme:right sidebar showing at bottom of page on right side

You can add certain stylesheets for certain versions of Internet Explorer, in the header.php file. Look at http://www.quirksmode.org/css/condcom.html for guidance.

Play around with these styles on line 89 of style.css,

#content {
margin: 0 34% 0 7.6%;
width: 58.4%;
}

Reduce the numbers to see if the sidebar returns to its correct position.

After you’ve found the problem, revert back to your original style.css styles, and instead apply what you now know to be correct styles to a child-theme CSS file.
http://codex.wordpress.org/Child_Themes

If you use child themes, you aren’t in danger of messing up your site. Your child theme CSS file will only use additional styles, which can always be removed.

If you’re considering using a child theme CSS file, add this code

#content {
margin: 0 34% 0 7.6%;
width: 58.4%;
}

Then mess around with the numbers in that child theme CSS file (not the original style.css file).

You can use browser testing websites such as browserstack.com to test your website.

If you mess around with line 89 of style sheet, reducing them, you might be able to see if those versions of Internet Explore require smaller widths to include the sidebar’s correct position.

Take into considerations I made with child themes and reverting changes to original style.css documents when finished.