To have a domain (sapphiregirl.com) that point to the directory where you have deployed the slave site, you can just update your cPanel (or equivalent HTTP Server admin tool) to declare the domain with the "document root" that point where you have deployed the slave site.
You also have to add the "sapphiregirl.com" domain in the slave site definition to make it synonym of the existing slave site definition.
Also add the variant "
www.sapphiregirl.com"
So yes you can do that.
In terms of SEO, in general, we recommend to deploy the slave site into a specific directory like presented in the tutorial video 0.b slide 21-24.
The objective is to avoid that website with "domain.com" is discovered as a subdirectory of another domain. (here your master).