"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".
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.
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".