Sister Company

because objects are generally referred to as female. thus sister company

because when 'sister' is referred to an object, it often carries a subservient connotation

Don't tell my sister that! The word sister just happens to carry the additional adjectival meaning of being of the same kind.

Guessing at word or phrase etymology is pretty dangerous because it is almost always wrong. Unless you do some serious research you're just grasping at straws. With that said, the phrase "sister (thing)" isn't a recent invention; apart from "company" there's "city", "ship", etc. "Sister ship," meaning a ship built similarly and concurrently, is the earliest usage I could find. The phrase stuck and thats the way it is. pseudo is probably right in that ships born at the same time are sisters because they often referred to as female.

As for the anti-female stance that pseudo talks about, I don't see it that way. I call my car a she not because I want to dominate her, but as a term of affection and personification. I love my car too much to call her it. :) Why "her"? Because I happen to like girls. :)

Also, personally, when I think of "sister" I think of older sister because thats what MY sister is :)
It's not all sexism...

This is old English. Things like countries, houses, vessels, buildings, etc are generally referred as feminine because the characteristics of these items bring to mind fertility and shelter. But with more emphasis on using non-gender terms, everything is referred to as 'it', so most companies prefer to use the terms "affiliates", "associates", "subsidiary" or "branch" nowadays.

It's biblical. Sisters get along. Brothers kill each other.

I used to always ask questions like this about the French language, when objects were given a "sex" during reference. I'd imagine if you figure out that language, the answer to this question would soon follow...

Capitalism originated in the Renaissance. The very first corporations were formed to finance sea voyages. Each company was formed for a single ship voyage and dissolved at the end. Since the ship basically was the company, the usage of "she" for the ship transferred to the company itself. Thus we now have sister, daughter, etc. companies.

It's also usual to think of things that give rise to other things as "mothers" that give rise to children. When the "children" give rise to others in their turn, they become "mothers." Therefore they had to have been "daughters" of the first "mother."

The expressions on companies equal in standing ubderneath the same umbrella vary from different cultures. In china, it's always said "brother companies" refer to companies equal in standing and very close to each other. It's the same with "brither school, brother association" cause China used to be a male-dominated country in the past over 5000 years.