No, the Japanese consider it to be canon and Kishi did approve it/draw illustrations for it. The novels are meant to fill in the gaps Kishimoto left behind.
If you did that, you'd contradict Kishimoto's own words back in 2012 where he answered that Itachi probably did have a gf since he was popular with girls pre-massacre.
ot: He never had a lover. And that girl wasn't Shisui's sister or close to him. She was a friend of Itachi's and they both felt some attraction (her's being very blunt and Itachi's being subtle).
She made it pretty clear she liked him. In contrast, Itachi was pretty much asexual because he was so focused on being a shinobi. He caught himself checking her out once but then shook his head and thought to himself that he was a shinobi, that was his duty, and there was no time for romance.
Throughout the novels, their practically non-existent relationship is teased by other girls he knows, his father Fugaku, and Shisui himself (who practically ships them a bit and finds Itachi being with Izumi to be cute). Itachi makes clear many times that he's single and not ending up with anyone. xD
But, on the night of the massacre he kills her first and since she was the most difficult kill for him (she was his friend and didn't do anything wrong like the Uchiha Police Force... but the entire clan was being hurled into suicide with the coup so it was a mercy killing by him instead of by some random Konoha shinobi). He gains resolve and continues with his mission. However, he literally makes her die with happiness by showing her a 70 yr Tsukuyomi where they date, him giving her a ring, get married, have ***, have kids, retire from being shinobi, their kids move out, and they grow old together while the village and Uchiha are at peace. Since that was both their desire. It's cute in a morbid sense.
