Complex sentence: Her older sister, Tabane Shinonono, is the creator of the IS, yet Houki hates her because six years ago, Ichika agreed to become her boyfriend if she could win the national kendo tournament of their grade.
Split into simpler sentences: Her older sister, Tabane Shinonono, is the creator of the IS, though their relationship is strained. Six years ago, Ichika agreed to become her boyfriend if she could win the national kendo tournament of their grade.