It's the main one that English speakers are probably exposed to; but even in this thread there are mentions of other languages and cultures where this or similar relationships have specific terms and more generic ones like "cousin" simply don't exist.
Grouping them all under "Chinese system" is... let's say culturally insensitive.
_That_ is a reason enough to not use the term from any specific language as a constant name; let enough to not expose that in the UI to users.