Solution:
This should work for you.:
RewriteEngine On
RewriteCond %{REQUEST_URI} !^/wp-content/uploads/sites/2/
RewriteRule ^wp-content/uploads/(.*) /wp-content/uploads/sites/2/$1? [R,L]
If the url doesn’t contain wp-content/uploads/sites/2/ then it will add the sites/2/ to the url