There may or may not be another male character who is hinted at being the FL's biological father. FL was trying to help the not-yet-revealed-as-bio-father find his wife and kid. This second description might be a different story though.
Not sure but this perhaps?
https://www.mangago.zone/read-manga/the_real_daughter_is_back/
It's really similar but not it. The sisters aren't actually related and FL's fiance was a support for her while FL and ML were growing up.
FL was trying to help the Duke find his wife and kid but neither of them knew the kid was her. FL also revealed her parents were lying to her sister about sending support to the family that raised the sis when sis was missing. ML and FL are very much into and supportive of each other.
Looking for story where the parents mistreated FL because their missing child came back. Like the parents wanted to give everything that belongs to FL to their other daughter. FL was okay with everything except giving up her engagement with her childhood friend. Her sister also wants to get along with FL and doesn't mistreat her like their parents.