When and why do you call it a sister-site?

Usually some sites are referenced as sister-sites by others, Why are they called sister-sites?

And what relation should both sites have to call each other sister-sites? Same authors? Same owners? In that case why not call it "our other-sites"? Is it because of the different content?

Solution 1:

Generally sister-sites are equal and owned by the same parent.
The term is also used for companies that have equal standing with the same parent company

The usage originally comes from shipping, where ships built along the same or very similar plans by the same shipping line were called sister ships (the Olympic and Britannic were the Titanic's sister-ships). Ships were and are called 'she' partly from poetry/superstition and partly as an influence of the genders in old English.

Solution 2:

Sister sites are similar in function, being variations on the same Intellectual property, service, product and/or experience, are usually owned by the same person or at least the same company. Though sister sites can be owned by separate entities that have an affiliation with each other.