Pretty vague, yes, but I can not remember much from it.

It had twins, a boy and a girl, where the girl is obsessed with the boy. The boy has a girlfriend, but the girl does whatever possible to complicate things.

I remember a chapter where he got hit by a car, when the sister told him to prove it.

It's a korean manga, from what I remember. That is about everything I remember. Hopefully someone can help me out.

could it be Flower of evil?

It is Flower of Evil, I believe. I just read it a couple of days ago.
The twins have jet black-ish hair right? And the girlfriend of the boy twin
is older/a senior in HS?

Yes, that is it.

Thanks for the quick reply's.