I read this manhwa a few months ago but I’ve been able to find it since. It’s a regression manhwa. The female lead, her mother-in-law, father-in-law and little sister-in-law all regressed back to the time before the female lead married the male lead. The female lead is very sickly and took the place of her sister in the marriage because her family hates her and she normally stays locked up. The female lead has blonde hair; the male lead and the in-laws have black hair. The male lead did not regress and does not remember the female lead like his family does.
Edit: Female lead has a father, sister and mother. Male lead has a father, mother and younger sister. It’s a historic manga with nobles and what not. The male lead lives in the super cold north and after the female lead and the in-laws regress…the in-laws send a marriage proposal to the female leads house and she ends up accepting the proposal because her family thinks it’s meant for her sister.