Your sentence is a little bit difficult to translate without knowing the context since おねえちゃん does not necessarily mean someone´s older sister. It can refer to any young girl who is older than the speaker and roughly translates to the title "Miss" when talking to strangers. 「いいよ。おねえちゃんにあげるよ」is probably a response to a question like もらってもいい？(Do you mind if I have it?) or 本当にいいの？(Are you sure?). Thus, the fluent translation would be something along the lines of "Yes, I'm giving this to you ^^ (Please take it)" If you're curious, the direct translation is "It's fine, I will give this to you, miss^^"

ぉわえちやん don't make sense. Do you want to say おねえちゃん? If so, this sentence means " O.K. Give old sister".

I think you must mean "いいよ。おねえちゃんにあげるよ。" おねえちゃん could be used to address an older girl or an older sister directly (the same as "you") or referring to someone else within a conversation. Depending on the context, it could mean: "Sure! I'll give it to you (big sister)." "Sure! I'll give it to you (older girl)." "Sure! I'll give it to my older sister." "Sure! I'll give it to her (that older girl we know)." The ”いいよ” is a response to a "～てもいいですか” (may I...) question.