Wherever you go that's where I'll follow,

Katsuki is not Genya. Katsuhiro knows this much, but yet he couldn't help but compare to himself, and compare. It was his only way to cope and justify himself that he wasn't replacing Genya. Like he could ever replace Genya, he couldn't.

Katsuhiro was an odd big brother, but Katsuki felt like his brother hadn't been genuine with him. Katsuhiro always seemed like he was holding back, even if he reassured the younger that everything was alright, but Katsuki knows it's far from alright.

Katsuki hated lies, and he hated it coming from his big brother the most.

.

or in other words, Sanemi is reincarnated as Katsuki's twin older brother and struggles to grasp the fact he won't ever be seeing Genya ever again.
(spoiler alert, he takes his time coming to accept his new life and brother)