Create a site with the same domain but in a different directory:

mastersite.com
/var/www/html/master/

mastersite.com/slave
/var/www/html/master/slave

when trying to access the slave site, show me the master site.

When a slave site display the master website, this is either because the URL is not defined in the JMS slave site definition
OR that you had an error when saving the slave site definition.

You have to provide all the variant of the URL.
For example, if you want to access the slave site with WWW, without WWW and with HTTP and HTTPS, you have to provide all the variant

mastersite.com/slave
www.mastersite.com/slave
mastersite.com/slave
www.mastersite.com/slave

and when you want to access via the port number, you also have to add the variant with the port in the URL like
mastersite.com:80/slave
www.mastersite.com:80/slave
mastersite.com:443/slave
www.mastersite.com:443/slave

When you don't know exactly the URL domain that is received in this directory, you can always use the "hello.php" test file that you can find in the tutorial video 7.

When you have an error when saving a slave site definition, JMS does NOT rebuild its "master index". So the list of domains are not recognized.
When JMS is not able to find an exact matching with a URL, it displays the master website.
This is the symptom that you have described.

I hope that all the cases listed above will help you identifying in which case you are to fix it.