"My little cousin is so jealous!"
However, depending on the context and the nuance you want to convey, other options might be better, such as:
* "Oh, my little cousin is so jealous." (More expressive)
* "My little cousin is such a jealous little thing!" (More playful)
The word "celocito" is a diminutive, making it clear that the cousin is young. The best translation will depend on the specific situation and your intended tone.