I've read the Q&A and related articles and am still confused. The sentence is: My sisters are often late, I always wait for them for two hours. When I apply the information in the lesson's Q&A, I ask myself: "For whom do I wait? For my sisters." Therefore sisters should be an indirect object. However, the quiz gives the direct object pronoun, las, as correct. Which is it and why? (Also, it would be helpful if you edited the questions so the correct answer to this appears consistently.)
ps: it looks like Marcos and Lisa, below, are asking the same question in slightly different ways. Perhaps you can address all 3 questions as a group. Thanks!