"Her cousin is a famous actress."
Translation:בת הדודה שלה שחקנית מפורסמת.
Consider that your mother's sister is your aunt, but so is your mother's brother's wife. The daughter of either is your cousin.
Theoretically you should have all four forms: בן דוד, בת דוד, בן דודה, בת דודה. In actual use, we usually pair the son with the uncle and the daughter with the aunt.
I don't think people will look at you funny if you use בת הדוד, but you won't hear Israelis say this much.
That's the thing with smichut:
The correct way is that the nismach does not get the definite prefix, and the somech does: בת הדודה
The incorrect way, which way too many people use is that the nismach gets the prefix and the somech doesn't: הבת דודה
Using the prefix on both is both wrong and not done.
If you check the tips and notes, you will find this message:
In Hebrew the word for a male cousin is literally “son of uncle” (בן דוד), and the word for a female cousin is “daughter of aunt” (בת דודה). Technically, “son of aunt” and “daughter of uncle” are possible, but in effect we like to group the genders together.