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.
- [素敵]{す・てき}な~
- [愛]{まな}~
- [愛]{め}でたし~
- [愛]{いと}(お)しい~
- [親愛]{しん・あい}な(る)~
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.