Jose is Ana's son. Ana has one brother, Carlos, who is married to Marta. Carlos and Marta have two children, Eva and David. How are Eva and David related to Jose and Ana, and how might Marta be related to Ana? Respond very briefly.
Eva and David are the children of Carlos, who is Ana's brother. This makes Eva and David Ana's niece and nephew. In relation to Jose, since Carlos is his uncle (brother of his mother, Ana), Eva is Jose's cousin and so is David.

Marta is Carlos's wife and therefore the sister-in-law of Carlos's sister, Ana. By marriage, Marta is Ana's sister-in-law. She is not related to Ana by blood, but instead through the marriage to Carlos.