"If you hadn't said that you were sisters, I'd never guess that you were twins."
- "If you hadn't said that you were sisters, I'd never guess that you were twins."
"If you hadn't said ~" 「〜言われなかったら」
"if ~" 「〜たら」
"that you were sisters" 「貴方達姉妹は」
"I'd never guess that ~" 「〜って判じない」・「〜ってバレない」"I'd never have thought that" 「〜って思わない」も言えます。〜って想像付かないと言う意味を表します。
"that you were twins" 「双子って」
People wouldn't be able to tell you two were twins unless they were told.
「People wouldn't be able to tell」は「人々は気づけないだろう」という意味です。「tell」はここでは「見分ける」や「理解する」という意味です。
「you two were twins」は「あなたたち2人が双子である」という意味で、具体的な関係を指しています。
「unless they were told」は「言われなければ」という意味です。条件を示す表現で、「~しない限り」という意味があります。