I don't get the whole yandere thing either.
Ui is obsessed, but far from insane and if the last chapter is anything to go by, she's even more mature than we know her for.
She'll probably still support her sister if she needs to, but I don't think she'll end up stalking her. It's in both girls' interest to properly grow up and as things look now, Yui is well on her way, and once Yui has truly grown up, so will Ui.
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