It sounds awkward because you're used to putting these sentences into the active voice. "Her parents love his sister." The whole point of this exercise is to learn about how to use the passive voice. "His sister is loved by her parents." It's used a lot but you just don't really notice it.
Active: "The dog ate the homework." / "We held a meeting." Passive: "The homework was eaten by the dog." / "A meeting was held."