Wow this mangaka sure dislikes the oldest sister huh? She and her partner have had like one or two chapters. This series might as well have just been about three sisters instead of four. Kinda weird to axe off a supposed part of your story so early on.
What are you talking about ? All sisters had 1 chapters per part. See the numbers at the end of the chapter, we're at part 8 and the next chapters will be the oldest and second one chapter. There is no axe at all, if you check the chapter list, you will see that part 6 and part 7 chapters for the eldest was one after the other which explain why we don't have Oldest chapter yet.