It kinda reminds me of this manhwa, but it's probably not it
https://www.mangago.zone/read-manga/cheerful_countess_sisters/
No, it's not ;------;
What hair color has FL?
I don't remember, i only remember the sister and the knight cause i got bored of the main history.... But i think it's blond too
Then what was story about, that you got bored? I read a lot of manhwa, but your description is just too short on hints.
Found it... The name was I Dont Love You Anymore
I don't remember the name, but it's an isekai shoujo
... The FL has a little sister that runway from home and stay with her a few days and fall in love with a knight of the ML, he has red hair and the little sister has blond hair...