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Think about it. Most of her actions seem to be purposely geared towards affecting Eiri and Shuichi's relationship. It was her that helped Eiri say the right things to win Shuichi back. When Shuichi kissed Reiji due to his anger at Yuki, Yoshiki was the first to pop out of literally nowhere a smack him around. This lead to her agreeing to help them both by dressing up as his brother Yuki Kitazawa and forcing a confrontation with Eiri.
After meeting Eiri for the first time in New York, and seeing how Eiri was still consumed by guilt over the death of Yoshiki's brother, which Yoshiki wholeheartedly agrees he deserved, she may have decided to try whatever she could to help Eiri overcome his past and try to prevent Shuichi and Eiri (especially Eiri) from screwing up their relationship like Yuki (her brother) screwed up his, and help the two grow closer.
This would mean that Yoshiki knows exactly what she's doing, and her floaty personality simply makes it seem like she doesn't know what's going on.
This is explored even further in the next WMG . . .
And Yoshiki is planning for this to happen.
In Gravitation Ex, after Eiri and Shuichi leave the cemetery in New York, Yoshiki says, "I'm doing a cruel thing, aren't I?" This hints that she knows the effects Riku is having on the couple, especially Eiri, but still plans on leaving him with them since she believes it will be what's best in the long run. It may be possible that this is when she called in the favors owed to her by Shuichi and Eiri. Perhaps she's hoping that this will help Shuichi and Eiri grow as a couple, and also act as the final step in helping Eiri completely move on from his past.
Even if they don't adopt Riku, that doesn't mean that simply dealing with him for a week isn't meant to serve a greater purpose. A purpose that Yoshiki wanted to happen.
Seguchi Tohma is an enigma. He proclaims Yuki to be the one person he loves most in the world—whilst being married to Yuki's sister Mika—he spends more time with Yuki than with Mika, and he threatens Shuichi's career and life in order to force a breakup between the two of them. However, none of this really indicates that Tohma loves Yuki in a romantic sense. What this might actually be is overcompensation on Tohma's part. It was Tohma who introduced Yuki to Kitazawa, and so Tohma was indirectly responsible for the trauma Yuki suffered at Kitazawa's hands. Tohma recognizes this, which is why he tries to insert himself into Yuki's life as much as possible and freely talks about how much he loves Yuki. He feels immense guilt over what happened to Yuki and is terrified that the younger man will be hurt so horribly again, so he does what he feels is in Yuki's best interests. He sees that a negative love destroyed Yuki emotionally, so he tries to shower Yuki with what he feels is a more positive love. And he recognizes that Shuichi has a personality type Yuki had as a teenager, and that being around Shuichi at first is very stressful to Yuki. So Tohma threatens Shuichi, not out of jealousy for their closeness but because he feels he's protecting Yuki from getting hurt again.
In a way, Tohma might also be feeling trauma over what occurred between Yuki and Kitazawa, as he'd walked in on the bloody aftermath. This would explain why his motives come off as twisted and why he unconsciously hurts his wife with his obsession with Yuki. But this doesn't mean that he doesn't love Mika or that Yuki is his true love. He even begins to realize his own twisted nature when Mika almost miscarries their child. But despite his eccentric behavior around Yuki, and his proclamation that he loves Yuki most, when Tohma looks at Yuki he doesn't see a romantic partner. He still sees the sixteen-year-old boy that he put in a bad situation and he overcompensates his guilt trying to make Yuki feel loved...in just a familial fashion.
