Actually, to me (and I think this is a strength of the story) the big sister only goes from "jerk" to "ambiguous". Yes, a couple of little sister's recollections of having things taken away are actually recollections of having things not taken away. But she still treats her routinely like a servant, and she still hangs with nasty vapid backbiting friends and lets them nastily backbite her girlfriend. She turns out to be not so bad, but I don't think the story is making her out to be totally sweet and wonderful under a tsundere facade, just a realistic person with good and bad points (and maybe capable of being recalled to her better self by the love of a good woman). IMO people are forgetting this because we tend to be too easily swayed by the most recent snippet of information.
Younger sister would probably have been better suited to Kuga-san, but she's out of the running because she's younger (and 'cause she met her second, which is fair enough). It's good her older sister has a nicer side, but the one who brought it out is Kuga-san. To me that second-to-last page, with the message saying "sorry for stealing your big sister away from you" is doubly ironic--on one hand, that's the opposite of her problem, rather she feels like her big sister stole Kuga-san away, and on the other, Kuga-san (with her gentle, generous influence) if anything gave her back an older sister she hadn't had for a while.
It's a great story, but I do find it bittersweet, and I don't think it should be disqualified just because the girl in love is young. I remember how it was--when I fell in love at 13, it was as hard a fall as I ever had in my life, ripped me up and made me as sad as anything ever did. It wasn't some little thing just because I was young, and I think that's true for lots of young people.