Korean · Pronunciation Guide
eon-ni. The two ㄴ letters across the seam merge into one long held n, so the middle of the word takes an extra beat.
eonni
Two beats joined by a geminate. The n really is about twice as long as a single one, and that length is the main thing separating this word from a hypothetical 어니 with one quick nasal in the middle.
Listen: 언니
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제 언니는 선생님이에요.
Je eonnineun seonsaengnimieyo.
My older sister is a teacher.
Listen: 제 언니는 선생님이에요.
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제 언니는 선생님이에요 puts two more nasals right after it in 선생님, so the sentence is unusually resonant -- and 선생님이에요 then moves its ㅁ into the copula, giving ni-mi-e-yo.
English speakers produce a single n and the word comes out short, so hold the contact through both syllables. The first vowel is the other issue: 'on-ni' with rounding is a common distortion, and ㅓ needs the lips completely slack.