YOU SAID:

my cat is lying on my sisters backpack with a smug look on her face

INTO JAPANESE

私の猫は彼女の顔に独善的な表情で私の姉妹のバックパックの上に横たわっています

BACK INTO ENGLISH

My cat is lying on my sister's backpack with a selfish look on her face

INTO JAPANESE

私の猫は彼女の顔にわがままな表情で私の姉のバックパックの上に横たわっています

BACK INTO ENGLISH

My cat is lying on my elder sister's backpack with a selfish look on her face

INTO JAPANESE

私の猫は彼女の顔に利己的な表情で私の姉のバックパックの上に横たわっています

BACK INTO ENGLISH

My cat is lying on my elder sister's backpack with a selfish look on her face