Forgive my ignorance, but I am still a newbie on these matters.
A parent site is a website, and it is synonymous with a main site, correct?
Whereas a sister site is a site closely related to a parent site, a type of “spin off” that successful TV series sometimes generate. Correct?
For example, if I've understood correctly, EL&U (English Language & Usage) would be the parent site while ELL would be its sister site.
But why isn't a sister site called a daughter site, or a child site instead?
A ‘parent’ generates children, not siblings.
Amazingly, it seems the expressions parent site and sister site are not listed in any online dictionary.