Chinese has distinct words for relatives of the same generation (siblings, cousins) who are older or younger than oneself. Like "big brother" and "little brother" in English, but you always include the big/little part. "Younger" specifies that this cousin is younger than the person whose contacts book this is.
Female - Chinese has separate words for male and female cousins
The rest of the word salad specifies a relationship that is approximated by answering the question "is her (maiden) surname the same as my (maiden) surname?" in the negative — either she is a daugher of one of my mother's siblings, and doesn't share my surname because I have my father's surname, or she is my father's sister's daugher, and thus has her father's surname, which again is not the same as mine.
In English, your male siblings are "brothers", and female siblings are "sisters", regardless of age.
In Chinese, we differentiate between younger and elder siblings. Your elder brother is 哥哥, younger brother is 弟弟; elder sister is 姐姐/姊姊, and younger sister is 妹妹.
A quick test in google would suggest that CNLabelContactRelationBiaoMei doesn't introduce too much ambiguity (first result for Biao Mei is still about the correct term), so even without unicode it might be fine.
堂哥 male cousin older than me 堂弟 male cousin younger than me 堂姐 female cousin older than me 堂妹 female cousin younger than me
表哥 male cousin older than me 表弟 male cousin younger than me 表姐 female cousin older than me 表妹 female cousin younger than me
Anyway, "Kleincousin" and "Großcousin" don't imply the age of the cousin, but a degree of relationship (with regional differences to what is actually meant).
To refer to a younger cousin, I would just say "jüngerer Cousin". "kleiner Cousin" may be possible too (like "kleiner Bruder" for a younger Brother), but it sounds a bit like childs talk and it may not be immediately clear to everyone what is meant.
[0] https://de.wikipedia.org/wiki/Verwandtschaftsbeziehung#Cousi...