I'd wager Hikaru and Kosuke get married when they grow up while Hikari's Sempai dumps her and she doesn't find a guy interested in a long term relationship.
I'd be willing to bet that Hikari didn't actually have another date, and instead could see her sisters feelings written on her face, and decided to hook her up with her soon to be ex. To be clear I don't think Hikari was in love with Kou anymore.