Okay but consider: IDC acted the way she did in her first appearance because she kinda Knew DC had started dating the main five (not that he told her but she knew, big sisters know) and she wanted to like yknow 'assert her dominance'

IDC has to learn to let go but it's so hard

Because the main five may of had DC's heart but she practically raised him goddamn it, he was her twin, they didn't know what they've been through. She was still number one in his life.

(This all ignores the ending of course)