I remember 2 Kdramas with somewhat this plot. took me a bit ot time to get the title. Went through mental gymnastics to get the title, I was finally able to remember the ML actors on said dramas, then went through their filmography. Anyway I am not sure if these are the dramas you are looking, the vague plot sounds to be the 1 you are looking for but I totally forgot most of the drama plot and setting, so good luck:
- Good Job Good Job - daughter-sister is very young 4-5y/o
- Here comes Love - Daughter-sister is college student and is very proud of her unnie