I think this would work, but there are several issues here.
「すばらしい」 is literally "wonderful", but it feels a bit too distant/impersonal/flaky to me. There are other words that would semantically satisfy, but I'm not sure how acceptable/correct they are to use when conveying your feelings directly to someone.
Also,「 いとこ」 has several kanji depending on the age and sex of the cousin. Since you mentioned your cousin is female, you could use:
- 従姉妹 (general female cousin)
- 従姉 (older female cousin)
- 従妹 (younger female cousin)
Lastly, while it's acceptable in (American) English to address someone close to you by their actual relation ("Hey there, cousin/brother/sister/friend!", etc.), I don't know if this is done in Japan. My gut says that that they do not, and would instead just use their name, or nothing at all.
So I guess this is more of an objective answer, but this question itself may be too opinion-based as it is.