Actually I think she chose Takahito. She didn't want to be without him and decided on the same university as him. And if you remember the ending, she was talking to a dark-haired little girl. I believe that was their daughter.
Haru to Tsunagu: At the end of ch 24. Did you use that selfishness … to tell him how you feel? Then when she thought he left for England she didn’t want him going without her.
Takahito to Tsunagu: But from now on, it’ll be just the two of us.
Got this from My Anime List Forum: Review of chapter 24:
Two years of time-skip and nothing seemed to have changed until right before their graduation and the day before the twins turned 18, Haruhito proposes to Tsunagu only for her to turn him down.
She eventually moved with Takahito to England to study in Oxford.
Well, with another time-skip some years later, at least we know she married Takahito and had a daughter with him as Haruhito became the King. A much better closure of the manga I would say.
I think Tsunagu's character got destroyed because the author was rushed to end the manga, so they had to skip a lot of development and finish with an ending where none of guys is chosen... A pity, this could've been a good story :')