Answer:
The pronoun that correctly completes the sentence is them.
Step-by-step explanation:
The sentence above contains a compound subject. A compound subject is one that consists of more than one noun (my father, my sisters). The individual elements in a compound subject are joined by a coordinating or correlative conjunctions.
When and is used to join the elements in a compound subject, the compound subject is treated as plural. However, when using or, either/or, or neither/nor, the compound subject might be singular or plural. Usually, if all the elements are singular, then it should be treated as singular. Yet, this is not the case. The NP "sisters" is plural, so the compound subject should be treated as plural despite of the fact that it has been coordinated by or. Therefore, the pronoun that correctly completes the sentence is them.