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.
EDIT
Amazingly, it seems the expressions parent site and sister site are not listed in any online dictionary.
Best Answer
It doesn't.
You've made an error in your classification.
ELU is not a parent site to ELL. ELU and ELL are each other's sister sites. They are on even levels of the SE architecture.
The main parent site of both ELU and ELL is Stack Exchange, though we rarely refer to SE as the parent site. The most common usage of parent/child on SE is to refer to sites and their metas.
ELL is the main site and each site has a meta site. SE also has a meta site. To help differentiate from the main SE meta, the site metas are called "child metas".
So, ELL is the parent site of ELL Meta, which is the child site.