There are 2 answers to this question based on the punctuation, as in sisters pleural or sisters singular with an apostrophe, such as sister's family, meaning possessive.

So,

With sister's possessive, as in 1 brother/sister/family unit, thus the answer is 1.

With sisters pleural then collectively there are a total of 6 brothers/sisters/family unit, thus the answer is 6.

Therefore the true answer, since the sister's possessive is missing, then the answer is 6.