I used to walk with my little sister to her elementary school on my way to junior high school because our school routes were the same.
この場合、次のような言い方ができますよ。
ーI used to walk with my little sister to her elementary school on my way to junior high school because our school routes were the same.
「私と妹の通学路が同じだったので、中学に行く途中、妹の小学校まで一緒に歩いたものだ」
to walk with my little sister to her elementary school「妹と妹の小学校まで歩く」
school routes「通学路」
ご参考まで！
カナダ 
