Headcanon: Jack in RWBY didn't get to return to the Arc household, until both his parents passed away in a freak Grimm Attack and his younger sister became the Matriarch of the Clan. Juniper Arc had matured both in body and behaviour, not to mention recently married through an arrangement by their late father.
But he didn't know how much she'd changed, till he sat in their father's study, with his naked sister in his lap, as she demanded he help her secure the Arc Bloodline. Her "husband" was nothing to her, so long as Jack was alive, well and able to breed her, just as she has always dreamed of.
"I've missed you so much, dear brother~" Juniper had a desperate, crazed look in her eyes as she gave Jack's huge cock a lewd lapdance, smiling at him. "So, please, now that you're back, slamfuck me into oru father's desk, treat me like your little whore, like i'ma dirty, depraved, brother-loving slut who wants to be knocked up, and breed me~"
Jack was a bit surprised by her eagerness (he thought she wouldn't be lustful for a couple of days, and instead she was asking for him to creampie her her "like her pathetic husband never could" and keep the Arc bloodlien strong, mere minutes after meeting him for the first tiem in years. But he was never one to refuse his hyper cock anything, especially when it was coming from a woman as gorgeous as Juniper was now. So breeding her, bending her over their father's desk as he made her his for the next hours, was exactly what he did, only stoppign when she passed out, her womb full to bursting with his potent cum.