I think the motive is pretty clear, she’s lonely and fears that Jun branching out or having connections will leave her alone again.
If you take in everything we’ve seen about her since ch 1 it all fits, we see her in the beginning as some sugar mama who’s clearly been around the block gearing up for another quick fling but Jun making himself vulnerable to her gives her exactly what she wants, purpose, a place in some else’s life and more specifically as Jun’s “mommy” which is something she places heavy emphasis on. She yearns for this so badly she literally tracks him down and “marries” his father just to get to him and immediately starts imprinting on him, actively providing the comfort he desires while perverting it enough to make him focus on her as opposed to his own happiness and how he could be happy with other parts of his life, she actively ignores all his attempts to address his trauma since those are things she uses to manipulate him but is more than happy to push him into the arms of a girl who she knows likes him knowing full well she’s filling his head with sex and essentially setting him up to ruin his relationship with his childhood friend one way or another.
This isn’t something she’s doing on a whim either, she’s actively stalking him and has been since the first day she entered his home (I’m positive saki “conveniently” meeting up with Jun on his way home was definitely her fresh off of stalking him at school but even if you don’t think that she definitely was stalking him and Riko when they went out shopping for a gift) and I’m sure his father suddenly leaving for Singapore for who knows how long was part of the plan too, she’s isolating Jun so nobody can take him away and is using physical intimacy as a means of drawing him in, he gets to be happy, she gets to be his “mommy” and nobody gets in the way like they did after she met him the first time.
That or Jun reminds her of her little brother who she used to bang, idk
