"Sudden forgotten love rival out of nowhere" is already one of the most stupid cliche tropes in romcoms, but here it looks even more stupid... and I don't even care if she is actually his sister, or whatever, this is just braindead writing at this moment. Why the hell authors cannot simply write about relationships without adding unnecessary "default" tropes like forgotten love rivals, misunderstood siblings, "accidental pervert" situations and so on? God, I miss things like Horimiya and Oregairu...