You know, when I first read this (way back when the Onsen arc was new), and then along with all the updates, I was kind of on Uta's side.
But I just re-read the whole manga, and man, she is such a selfish child.
Cutting someone you love out of your life completely (which is what I infer by the use of the black word balloon Ch. 25, Pg 27), someone who clearly loves you back, just not in the same way, someone whom you admit brings you joy and happiness because, what, because it hurts that you can't have more?
I really wouldn't attach too much blame to either of them. It is a difficult situation, Uta is fairly mature, but she is still young and inexperienced, still in school. She doesn't really have much choice about her mom taking her away, so to a great extent she is probably just trying to make the best of the situation - she didn't come up with a plan to move away, she just chose what she thought was the best path of action with the situation she found herself in.
Kaoru was being a "selfish child" too. Kaoru was stubbornly refusing to accept that things needed to change between them, refusing to acknowledge Uta's feelings and the pain her denial was causing Uta. It wasn't sustainable, so something had to give, Uta is trying to protect herself, but she is also trying to do the right thing. If Kaoru had engaged, Uta probably wouldn't have felt the need to cut off contact entirely. It is unlikely she realises how much Kaoru relied on her (Kaoru didn't either), so it isn't really fair to blame her for Kaoru's crisis (it wouldn't be a fair load to drop on a teenager anyway).
Conversely I don't think it was entirely wrong for Kaoru to accept the support Uta freely offered over the years, but she does need to be considerate of the cost for Uta (especially since Kaoru is the adult). Yet, when Uta reached out and tried to explain, she went into hard denial. If she is serious about being sisters, she needs to push down her own discomfort to help Uta. Ask Uta about her feelings, talk it out, and ask how she can make it easier (maybe lead the way by being honest about her own needs, and thanking Uta for what she has done up to now).