I Don't Understand What My Sister's Friend Is Thinking (妹の友達が何考えてるのかわからない, Imōto no Tomodachi ga Nani Kangaeteru no ka Wakaranai) is a Japanese manga series written and illustrated by Rei.

Written and illustrated by Rei, I Don't Understand What My Sister's Friend Is Thinking was serialized on Comic Smart's Ganma!

[2][3] Its chapters were collected by Ichijinsha into three tankōbon volumes released from February 26, 2021 to July 5, 2022.

[4][5] During their panel at Anime Expo 2022, Mangamo announced that they licensed the series and added it to their app.

[6] The series was nominated for the sixth Next Manga Awards in 2020 in the web category and was ranked 10th.