The mix story path with Yui and Charlotte makes me excited for other mixed paths. Sasha and Layla would be very interesting to see, or even Sasha Maxy/Mary.
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I'm not certain the exact point, but I know Charlotte and her sister have to have gone through their grandmother's belongings. For Yui you have to have basically completed her story.
At some point when working (I think I decided to work as a waiter, not at all certain) Yui will stop by interested in helping Charlotte then another few days later Yui will appear in your room.
Not sure on the timing or exact requirements but that's what I remember