Either the Fishers or their daughter has seen the house for sale.
With the correlative conjunction either-or we decide the subject-verb agreement by the closest noun to the verb, in this arrangement, the noun daughter is closest to the verb to see, singular pronoun she is agreeing with has seen.
Other similar examples:
Either Susanna or her children are going shopping for Christmas this Sunday.
Either his brothers or David has broken the granny's porcelain vase.