the difference between unnie and eonnie is just the pronounciation,the correct spelling in korean is 언니 (eonnie) but the pronounciation is unnie.but there are two types of word in sister in korea this is the eonnie and nuna,eonnie is for girl and nuna is for boy when she call his sister.
@ninshii Actually @Ae-Ra0205 's response is misleading. The pronunciation should be the same, the difference is the Romanisation, or the system by which Korean sounds are approximated through transcription into the Latin alphabet.
The McCune-Reischauer scheme was officially deprecated by the Ministry of Culture in the year 2000. In the current scheme, called Revised Romanisation, the vowel /ㅓ/ is represented as /oe/ (and represented in International Phonetic Alphabet as /ʌ/, also called the open-mid back unrounded vowel.
Company names and family names that were Romanised prior to the changeover are exempt from the new uniform standards. Of course, there's no regulating the super-simplified spellings used by Western koreaboos. In Japan we use katakana to approximate Korean phonology and believe me it's even worse.
If you're a K-pop fan, it won't take you long to start picking up Hangul without even formally studying it. It's much easier than you think—and much easier than muddling through inconsistent- and inadequate Romanisations. Just pay attention to the characters splashed up on the screen when someone utters something funny or significant. I actually started by noticing patterns in the way the members of Twice spelled their names, starting with 모모—you might already know that spells Momo!
they all mean “sister” (older sister of a female speaker). They are the same word just spelled differently in English. However, like stated above it is 언니 in hangul The standard romanization for this hangul is "Eonni"