「、！」「」
"But we're siblings, aren't we?!" "In name only."
Literal
But, we siblings right? / Name-upon [at-では] [ね].
兄妹 (きょうだい) means specifically 'older brother + younger sister'; 姉弟 ('older sister + younger brother'), 兄弟 (generic 'brothers/siblings'), and 姉妹 ('sisters') pattern similarly — Japanese lexicalizes age and sex of siblings into single words. だろ is the blunt masculine contraction of だろう, here seeking agreement. 名目上 is the noun 'nominally / in name,' followed by では ('as for that state of affairs'). The reply is curt and deflating, hence the humor.