You guys are crazy, Mitsuki was the only one that actually tried to make Haruka face reality and actually think about what she wants from Yuu while trying to figure her own problems and being selfish enough to know what she herself wants
That's really how you saw it?
Here's how I saw it -> Mitsuki knew that Yuu (her sister) had a 'special' kind of relationship with Haruka, a person who she didn't even know visually, let alone on any personal kind of level. She only knew her through how often Yuu talked about her, and was curious as to why she'd had such an affect on her. She started to develop feelings for her from there on...
She caught the two of them kissing in Yuu's room and knew that their relationship was more than just friendship, and instantly tried to stop it out of jealousy. Even going so far as to get their OWN FRIENDS to spy on them and report back to her on what they were doing.
The second Yuu showed any kind of weakness and confusion between the relationship between Haruka and herself (which is natural for someone that age with a lot less experience than her older sister), she immediately took that chance to pull the rug out from under Yuu and try to take Haruka for herself.
No matter how you look at it, she's just a bitch and, an even worse sister.
/essay