Quite honestly same. Like I tried to understand her in the beginning, but I quickly gave up on that after the first few chapters of her being introduced. Since I too have an older sibling, but the sister in this manga never takes on an older sibling/mature vibe and I guess that what pisses me off when it is her little brother acts more mature than her at times.
Honestly, I want to see more of the older brother rather then her. Everytime, I see her I think "Great here we go again.... She's either going to threaten to beat him up because she wants to play or wants something or she just wants to beat him up" I get it siblings roughhouse. Me and my older brother do that. But she does that almost everytime she sees him.
I'll say this straight. I REALLY DON'T LIKE THE SISTER. She is selfish, doesn't listen, and peer pressures others, just to list a few. I could go on listing how much hate I have toward her just because of the type of character she is. I've come across other characters similar to her, but they were not as overbearing as her or had another aspect to them that balanced out all the other stuff. Like she makes me CRINGE.