Your older sister and younger sister look so much alike. I can't tell them apart.
Your big sister and little sister look exactly the same. I can't tell the difference between them.
ーYour older sister and younger sister look so much alike. I can't tell them apart.
「あなたのお姉さんと妹はそっくりで、区別がつきません。」
can't tell them apart で「どっちがどっちか見分けがつかない・区別がつかない」
ーYour big sister and little sister look exactly the same. I can't tell the difference between them.
あなたのお姉さんと妹は全く同じに見えて、区別がつきません。」
can't tell the difference between them で「区別がつかない・見分けがつかない」
ご参考まで！
カナダ 
