In this case, the 「ねーちゃん」in「従姉のねーちゃん」is a qualifier and not to be taken literally (i.e., "cousin's sister") since「いとこ」can mean any combination of age/gender when spoken, unlike the luxury of having a clear distinction when written in kanji.
'Eyo, Miko!If my older cousin ever saw you, she wouldn't be able to leave you alone!...that one's Riffle's?Mannerless boor! I am not to be referred to so off-handedly as "that one"!Mornin', Kei!...--choo!Older Cousin, Part IIWell, I don't figure she'd be any different....--choo!*nod*Ohh! You must be Miko's...?I'm Tiko! It's a pleasure!Ha, you're just like Miko...Huh, so if she'd react that strongly to Tiko, I wonder how she'd fare against seeing me?Older Female CousinMy cousin's the type who doesn't care much what it's like so long as it's cute.Are you coming down with a cold, you think?*chuckle*
