For the life of me I can't remember the title of a drama or any of the actors, but i can remember almost all of the plot.

- family drama

-family consists of mother, 2 brothers and one sister

-  one brother is the father of twins but he pretends to be their uncle

-mentally challenged girl loves her brother

Sounds like it might be Wonderful Days ?

The ML's (played by Lee Seo Jin) immediate family is his mom, his 2 brothers, and his mentally disabled twin sister. His hot-headed younger brother, played by Taecyeon, is the father of twins, but his mom claims/raises them as hers.

Yes that's it! Thank you so much.