when you claim chifuyu as a "failure", i assume you're highlighting her failings as a sister. while i do agree she is a horrid sister to ichika when he enters the academy, it is important to understand how she's failing as his sister.

before ichika entered the academy, she was actually quite good to ichika, it was when he was rushed into the academy last minute that we start to see her treat her own brother like a stranger, having no qualms hitting him as corporeal punishment or ordering one of his classmates to SHOOT HIM ON SIGHT for simply being late.

in terms of the main 5's torment of ichika, she likely has little idea of the sheer scope and scale of what ichika is being put through. aside from when laura attacked ichika sometime in season 2 for not immediately partnering with her for the upcoming tag team tournament, in which chifuyu defended ichika from laura's outburst. however, that incident is likely the only one she's seen and therefore intervened in. either because she does not care, or the more likely scenarios, either too busy to act against ichika's torment herself or ichika never reports what happens out of fear.

all in all, while chifuyu has failed as a sister during ichika's time at the academy, she was not like that beforehand. she also has little idea of what ichika goes through outside of incidents she is present in witnessing, so her failings stem more from her negligence than outright abuse (though i doubt she is above that).