I can only think of a small number of dramas involving an orphaned lead and siblings who look after each other to some extent, but I'm afraid none of them have exactly the dynamic you're looking for. But just in case:
In Come and Hug Me, the FL's parents are murdered and her adopted brother is her only family left. The brother and later her manager (she's an actress) look after her in the years after her parents' deaths. Once she reunites with the ML they play a smaller role.
In Hello Monster, the ML's brother is kidnapped after their father is murdered. When they reunite as adults he helps and looks after his brother.
In Who Are You: School 2015, the FL is one of two twin orphans. Her sister is adopted by a rich family, but she is not. However, her sister keeps track of her and looks out for her from afar. Then a series of events ends up entangling their lives.