We say "brothers" きょうだい, and "sisters" しまい. (However, きょうだい means "brothers and sisters" when we don't know their gender or when we say both, mixing up "brothers and sisters". では ありません means "not be" as "isn't / aren't". To make each word clear, I'll write the sentences putting spaces between words. この おとこのこたち は きょうだい では ありません。These boys aren't brothers. この おんなのこたち は しまい では ありません。These girls aren't sisters. I hope you understand this construction and I hope this was helpful.

-『きょうでい』？ほうげん(diialetto)かもしれません。ふつうは『きょうだい』です。 -『AはBでは ありません(A non e B.)』