I see a lot of Chinese female characters in English-language media named Mei or Mei Mei. I know there are different ways Mei is written in names, like 梅 and 美 (which is in my mother's name), and there's 妹妹 for little sisters, but I also see girls who seem to have Mei Mei as a given name, with no siblings in sight. Is this legit (and if so is there a most common writing) or is there a syllable reduplication custom for nicknames or something? Or did a bunch of lazy writers just go around naming everyone Little Sister because they heard it somewhere and thought it was cute?
"妹", originally was referred to the young female sibling and cousin, but later the meaning has extended to "young female" (檳榔妹, 西瓜妹... - a nickname for a young female who sales certain goods/item), and been used/included in the female's given name after the famous poet "蘇小妹" (though no prove).
"妹妹" retains the original meaning but the use as a formal name for a female (mainly due to the thought of the name will be inappropriate when she grows up). It has the following uses/meanings:
It is used as a generic "nickname" in addressing a little/newborn girl, who usually is the sole female kid, or the youngest girl, in the household, before she ever grows to the age that understands the meaning and feels belittled by the implication of "little (小)".
It is often used as/meant "the daughter(s) of a specific household " as in "鄰家的妹妹", or "李家的妹妹". In here, "妹妹" means "女兒", and usually refers to their young (more often pre-teen) daughter(s)".
It can also simply refer to a female and indicate her youthfulness in the group - "She is everybody's young sister (她是大家的妹妹)".
For the latter two cases, if the object is replaced by the name of a specific person or a specific person, then "妹妹" is strict "the young blood-sister of such person (她是王三的妹妹 or 她是他的妹妹).
Note, while all share the sound "Mei" in pronunciation, the words have different "tones" - 梅[méi,2], 美[měi,3], 妹[mèi,4].
“My favourite animals are actually red pandas. Before the whole project even started, before I was introduced, but the main thing is that my mom actually called me Mei-Mei before this project, ’cause Mei-Mei means little sister in Chinese.”
Like Rosalie Chiang spoke about here many Chinese parents do refer to their daughters as "mei-mei." Even though it does technically mean "little sister," it comes as an ellipse from: [谁谁谁]家的妹妹 - the "little sister" of the family.
Similarly you could refer to a only male child as: 弟弟.
The Chinese family unit is much larger - when younger cousins and friends start coming along they will slowly be graduated to big brothers and sisters.