It would seem like she has the potential to be a yandere. It would be a good possibility that she could be one, what with loving and supporting her all the way through, she has the right symptoms of such.
Yandere Ui is hilarious and quite possible being that yanderes are usually so unexpected after all they usually are kind to everyone and will help out just fine as long as nothing moves there target away from them. If say Yui was to move because of a member of the band to stay with that person(lets say Azu) for a long time then we would have a situation that could potentially cause Ui to go yandere
surprised sfxA killing aura!?over to you…sneaking up sfxI won't hand Onee-chan