Soooo, when does she become a yandere? Did I miss it?
I believe she's already crossed into that mindset, based on some more recent comments of hers. Because of certain variables though, Rui hasn't had to resort to any standard yandere activities:
She's probably the most beautiful woman in the school they attend, so BF has no real reason to approach any other female.
Any girl who might like the BF probably feels like they couldn't compete with Rui at all, so none approach him.
They have sex everyday now it seems, usually multiple times per day.
Rui dominates the flow of any conversation that they have, often shifting the topic to being about her.
If Rui gets even an inkling that BF is distracted by other women, she redoubles efforts to keeping his attention on her.
Mr. Boyfriend himself is completely in love with Rui and wants to marry her, and vice-versa.
Rui's family are totally onboard with their relationship, with her little sister especially working to reinforce it.
IMO, it would take an extremely traumatic event to cause the negative elements of Rui's yandere personality to surface.