Hi!
The manga I've been looking for is a romance that portrays a step-brother step-sister relationship, where the guy is a player before he falls hard for the girl. The art style is older, circa 20th century shoujo manga, and there are some steamy parts in it. Both of them have to separate later in the story for some reason (I think because the girl decides to go to university overseas, but don't quote me on that), and then when they meet again, the step-brother's gotten rich from stocks and the girl's cut her hair short into a bob. At the end of the manga, the girl also has a guy who's in love with her that she met at her university. It's been killing me trying to scour every manga database for the step-sibling relationship tag just to find it. I figured I'd try my luck here.
Any help would be great, thanks!

Idk if this is the manga that you're looking for, but maybe Forbidden Kiss? It's set in high school though.

I'm in idol hell T.T

It sounds like Kirai to me