How do I say have a sister but cousins in French?

This translates directly to "I have a sister but cousins."

Alternatively, you could use a slightly more nuanced phrase like:

* "J'ai une sœur, mais je suis aussi cousine avec..." ("I have a sister, but I'm also cousins with...") This emphasizes the cousin relationship.

* "J'ai une sœur et des cousins." ("I have a sister and cousins.") This simply states both familial relationships.

The best phrase to use will depend on the specific context and the nuance you want to convey.