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In this frame from ３月のライオン, Akari, the eldest of the Kawamoto sisters is being referred to as おねいちゃん rather than おねえちゃん. https://gyazo.com/e039edc3be657a4f1082dd9d152c0ae7
In this separate scene, it also sounds like she's being called 'おねいちゃん' than 'おねえちゃん' as well https://www.youtube.com/watch?v=suKLH_18tPI&t=59s
Is おねいちゃん commonly used and what would be the difference from おねえちゃん?
おねいちゃん is an incorrect or variant spelling/pronunciation of おねえちゃん. There is no semantic difference.
In my perception, the usage of おねいさん is sometimes found in manga among children or less educated people who otherwise use standard (Kanto) accent. The most famous user of おねいさん is Nohara Shinnosuke (although he also uses unusual words like オラ and speaks with a peculiar accent).
This type of "double vowel" confusion is characteristic of Shitamachi kotoba of Tokyo. However, this spelling isn't necessarily used to strictly represent a specific dialect. It might simply be used as an (often cute) alternative spelling to add flavor to a character.
I found an interesting analytical article by a language enthusiast specifically about this topic:
But there's probably no need to analyze it so strictly for most people. It's fine to recognize it just as an uncommon alternative of おねえさん.
おねいちゃん is just a misspelling of おねえちゃん. This word probably contains lengthened vowel from ane: ane → nē, so there is no rationale for ei in this word. (Compare the same process for ani → nī.)
A part of Japanese population pronounces ei within single morpheme as [e̞ː] instead of [e̞i]. There are much more words with originally ei than ee / ē, so some persons, by analogy, may use ei spelling even in cases where it is not legitimate.
Regarding words with correct ei spelling, pronunciation as [e̞i] or [e̞ː] may depend on dialect, and also pronunciation as [e̞i] is more likely in formal or careful speech.
Words with morpheme boundary in the middle of ei (e.g. 眼{め}医{い}者{しゃ} "oculist", 姪{めい} "niece" (this me is the same morpheme as in words 女{め}, 雌{めす}, 女{め}神{がみ}, 女{め}子{こ}, 娘{むすめ}, 嫁{よめ}, 姫{ひめ} etc.)) should have both vowels pronounced distinctly: [e̞i].
