RewriteEngine on
RewriteCond %{HTTP_host} !^$
RewriteCond %{HTTP_host} !^www\.domain\.com$
RewriteCond %{REQUEST_URI} !^/sites/
RewriteCond %{HTTP_host} ^(www\.¦)([^\.]+)\.domain\.com$
RewriteRule ^(.+) %{HTTP_host}$1 [C]
RewriteRule ^(www\.¦)([^\.]+)\.domain\.com(.*) /sites/$2/$3 [L]