Not sure if anyone’s done this, but I compiled a list of characters with siblings. I’ll be leaving out the Udagawa and Hikawa sisters in the list. If I missed any characters, let me know!
popipa
Kasumi: younger sister
Saaya: younger brother and sister
Rimi: older sister
Afterglow
Himari: older sister
pasupare
Aya: younger sister
Chisato: younger sister
harohapi
Misaki: younger sister and brother
Hagumi: older brother
Edit: so apparently in one of the card stories Hagumi said she also has a sister, but in another, she said she only has a brother. The stories were amended, thus she now only has a brother... R.I.P. Hagumi’s sister
Kanon: older brother