The 17th Princess Just Wants to Live

I found myself in the role of a supporting character who helped the heroine.
It was a brutal political romance fantasy where siblings killed each other over the throne, and I was the younger sister helping the protagonist.
Since the original story started three years later, I would just survive for those three years!
In this second life, I planned to live easily by sticking close to the heroine…
.
.
.
… or so I thought, but when I came to my senses, the villainous brothers had turned into doting fools over their little sister.
What on earth happened?
And male lead, why were you here?