I know that Yuna is older than Rikku, but I wonder who is the elder cousin sister?
don't know if this question makes any sense though :) because in my country, (elder brother's daughter) will be (elder cousin sister) of (younger sister's daughter), no matter how old they are.
of course, I need a reliable source :)

Welcome! :D

As far as I remember, it was never actually mentioned how older or younger Yuna's mother was in relation to Cid - all that was mentioned is that her mother was Cid's sister and that was it. Her name wasn't even mentioned, basically showing that all that was important to the plot about her was the fact she was related (ala, an Al Bhed), and hence, after marrying a Yevonite (Braska), gave birth to Yuna, who was essentially a contradiction between the two opposing forces. As far as I know the identity of Brother and Rikku's mother isn't even mentioned.

That is basically all that's mentioned on the subject on the FF Wiki, so I'm sorry I couldn't be more helpful.

Thanks for your attention :D
to tell the truth, I'm translating FFX wiki into my language, but my language has two separated pronouns for elder and younger cousin sister, but no pronoun for "cousin". I've searched through many English sources but haven't found anything. Just hope they mentioned it in a Japanese manual book...