This is fucked up but I wouldn't call it grooming. The older brother was forced into a parenting role which in itself is abuse. The younger brother is obviously traumatised for a couple of reasons and has no one else except for his saviour. They're heavily codepended and this sick intimacy turns into sexual desire. So again, I wouldn't call this grooming, it wasn't intentional on the older brother's part, rather it seems like he's trying to control himself but eventually gives in to temptation (big part of the fauxcest trope). They're both in need of therapy but neither is evil.
This author can write such different stories but they're all amazing
