Woah. Mitsuki haters e'erwhere. Not that I don't see why, I think it's just a bit preemptive and misinterpreted. Mitsuki didn't try to actively 'steal' anyone. Firstly Haruka and Yuu weren't officially (and still aren't) a couple, so there was only rivalry. And it wasn't backstabbing rivalry. Mitsuki did not ever try to make her sister look bad to Haruka, or threaten either of them, or anything like that. She only was up front and honest and confessed. And when rejected, she accepted it with a smile and gave her blessing to Haruka (and effectively Yuu).
I won't tell you to like her, since her love-triangle-ness, albeit a well done love triangle, is totally open to opinion. If you don't like her type of love triangle, that is totally fair. I will just ask you all to not call her a bitch or a bad sister or anything derogative, because she was anything but.
Plus, this series gives off happy vibes to me. So call me crazy but Rina x Mitsuki'll be endgame. Probably. (Nanami'd be proud of me)
I can't claim to know anything 'bout the future, but if you ask me saying that Kaede has an unrequited love for Yuzu is a bit short-sighted. Quite frankly both have given plenty of hints. Just be careful not to confuse Yuzu's straight man/woman role pointing toward lack of interest.
And I got no idea what SumiSumi will be doing. I love her as a fun side character personally, and if it does establish another love triangle I'd look forward to seeing how it plays out, since this series does a metric fuck ton of introspection and analysis on relationships of all levels and all kinds and what the difference between friendship and love actually is (/spoiler - not much at all) (/spoiler2 - but its still super goddam complicated).