Supposing that the italicized verb is <u><em>had gone</em></u>, then the tense is past perfect tense.
This past perfect tense is an action that has happened sometime in the past but before something new has happened. In the sentence, the word "gone" is the third form of the verb, thus making it a past tense. Then, the use of the word "had" which is the past tense form of the verb "to have" shows it is the "perfect" form of the verb.
Thus, the whole sentence "<em>Your sister had gone when we arrived.</em>" is in the "past perfect tense".
