"younger cousin (mother's sibling's daughter or father's sister's daughter)"

If you have an iPhone, you can check all of those out by going to Contacts.app, tapping "edit" > add related name > tap on the blue "mother" > scroll down to bottom and tap on "All Labels".

It doesn't. But after all English speakers are only a small (<5%) minority of the world.

This feature is exclusively for the CN market. Where they have a very specific work for each of those.

and it is probably to improve siri. So people can say "call 伯伯" and Siri knows *exactly* which uncle you mean. which is probably some killer feature that only android phones from huwai had before.