Options +FollowSymLinks
Options +SymlinksIfOwnerMatch

RewriteEngine on
RewriteCond %{HTTP_HOST} ^siteismin.com$ [OR]
RewriteCond %{HTTP_HOST} ^www.siteismin.com$
RewriteRule ^(.*)$ http://www.siteismin.com/forum/index.html [R=301,L]