It's pretty obviously a happy ending, if you take the epilogue in account.
Shou acknowledges her little sister for the first time in five years. I think it's implied that their relationship went south ever since Yuu recognised that Shou thought her as a nuisance, and actually taking the effort to greet her is a small, but symbolic gesture. Also, Yuu's classmates talking about the ghost (i.e Chiharu) hanging around the house implies that Shou managed to make contact with Chiharu and is actually buying a cellphone to contact her. Also, Chiharu saying that Shou 'grew her hair long again' pretty much spells out that they met after Shou got her hair cut. And that big ass smile from Shou in the end isn't something that ever happened through out the manga. You can't get anymore clear cut, really.
Also, while I rarely try to bring up realism as a defence for manga since it's usually a bogus one, I think this one deserves it. Shou was a completely broken person, and to her go through a dramatic change at the end of the manga would be, well, unrealistic. Speaking from experience, changing is an incredibly difficult thing for a person to do, and is a long term thing. You have to look at it from Shou's perspective. Shouting towards Chiharu at the train station, asking for a cellphone and saying 'hi' to her sister were most likely difficult, nigh-impossible things for her to do unless she really, really wanted them.
Overall, I'm satisfied