"Her cousin is a famous actress."
Translation:בת הדודה שלה שחקנית מפורסמת.
You can say Bat Dod (and Ben Doda) But Bat Doda and Ben Dod are more common than Bat Dod and Ben Doda.
I agree that these forms are not often used, but in truth they should be. The son of my aunt is my בן דודה. The daughter of my uncle is my בת דוד.
Sure, but unless your uncle or aunt are single parents, it doesn't matter. Your uncle is not just your mother's brother or your father's brother, despite what one of the sentences in this unit says. Your aunt's husband is also called your uncle.
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.