ーYou don't really look much like your younger sister.
「あなたは妹とあまり似てないね。」
You don't really look much like ... で「〜とあまり似てないね」
ーYou don't resemble your sister all that much.
「あなたはあまり妹と似てないね。」
You don't resemble ... all that much. を使って「〜とあまり似てないね」とも言えます。
ご参考まで！
カナダ 
