How does a "parent" generate a "sister"?

Question

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.

Answer 1

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.

Answer 2

What I say below is my opinion based on some experience and some reading and I have no citation to back me up.

First of all, I would like to point out that in biology, the chromosome which is going to divide (in mitosis) is called 'mother chromosome'. The two branches of the mother chromosome which are going to be pulled apart are 'sister chromatids' and after division the sister chromatids become 'daughter chromosomes'.

This mother, daughter, sister terminology derives from the fact that when biology started, reproduction was considered a feminine subject and therefore anything that divides was named(mostly) in female terms.

The 'parent', 'sister' websites must also have come from the same analogy. Also, here the parent website is the main website and sister websites are not exactly children of the main site but rather a branch of the main site, just like chromosomes which divide into branches although the branches remain attached rigidly at the centromere.

Here are two links which are not very reliable websites but support my claim nonetheless - link #1 and link # 2.

Answer 3

There are two terms − one, "subordinate" and, the other, "coordinate".

In the instant question, the second website stands in a coordinate relationship (and not in a subordinate relationship) with the first one. In coordinate relationship, both relatives stand on the same level. As such, none is a "parent" of the other and none a "child" of the other; both are "sisters" of each other.

Actually, the instant question proceeds from a mistaken premise which is as follows:

Two websites stand on two different levels of height, where the second emanates from the first and thus stands on a lower height. Then why is the second website called "sister", and not called "child", of the first one?

As both the websites are in a coordinate relationship with each other, none is called a "parent" and so the instant question − asking why the other website is called "sister", and not called "child", of the first one − is a non-question.

Answer 4

There are parent sites, child sites, and sibling sites (each can have their familial gender-themed version, like sister or father site).

Stack Exchange is the parent site for both ELL and ELU. ELL is a sister site (or brother, it doesn't matter) to ELU, and vice versa. This makes sense; they're both part of the Stack Exchange network. ELU and ELL are both daughter sites (much more common than "son site", especially in biology) to Stack Exchange.

Offspring are suboordinate to parents, not siblings.