Wonder why the eldest sister was left out... hmm 🤔. Rather convenient one might thing.
They have basically been besties since they were kids and they've never met each other's families or gone to each other's houses? Weird.
Besties is a strong word. They've been classmates. Actually adjacent classmates (in different classes) and not even always friends since they were kids. As it's been described they only recently started hanging out after school.
Ibuki especially seems to have gone through multiple stages. Tomo was always friendly, but Ibuki seemed really abrasive in elementary due to the clash between traditional gender roles and how she didn't fit them. Then we haven't learned much about middle school (as of this serialization anyway; idk if the webcomic went more into), but that she was forced to wear the girls uniforms seems a little telling. It seems like Ibuki didn't really embrace being herself and open with others until high school.
Anyway it's hardly weird. I have close friends whose homes I've never visited and people I'd interact with constantly at school that I didn't go out of my way to do stuff outside of school with.