The sentence that contains an error in pronoun use is Sentence 1.
The sentence states "Robert and his brothers always help his dad maintain the family farm every weekend." The pronoun "his" should refer to a noun that has already been mentioned. In this case, it should be "their dad" instead of "his dad," as the sentence includes both Robert and his brothers.
So, it should read: "Robert and his brothers always help their dad maintain the family farm every weekend."