Per the timeline, Justine was born about 17 years before Storm Front. Grave Peril was about a year later, so Justine was about 18 when Harry met her. Which means, when Harry killed Justin, she'd have been about 2.
Also, IIRC, in GP, Justin said her parents had placed her in "special schools" really institutions/mental hospitals throughout her life, which would suggest that Justin wouldn't have had "access" to her.
I hate to poke holes in theories, but this gnawed at me. Sorry.