The sentence that correctly uses a semicolon is sentence 1. A semicolon can be used to link in a single sentence two independent clauses that are closely related in thought. Moreover, when a semicolon is used to join or more parts in a sentence, these ideas are given equal rank. In 1, the first clause states that the speaker has found his room totally messe up and then the second clause is about the person who might have messed it up (her/his sister). As you can see, the two clauses are related in thought.
