Answer: Ana blocks the seat next to her because she doesn't want her brother to sit next to her.
Explanation:
From Ana's actions, it would appear that she did not want her brother to sit next to her on the bus. First she places her bag on the sit opposite her with the sole intention of preventing David from sitting on that seat.
When David does come up and insinuate that she move her bag so that he may sit down, she responds by placing her hand on the bag to emphasize that she does not want him to sit down there.
