Brothers / Family Members Fall for the FL

Uncle and nephew

The FL lives as a man. The ML and his sister both fall for her.

Uncle and nephew

The ML and a past / alternative timeline version of the ML both fall for the FL. They're a bit like twins. The FL is from modern times and her soul takes over the body of a girl in the past after that girl commits suicide. The ML's brother was in love with that girl and refuses to accept that she's gone. He tries to marry the FL. 

The FL is from modern times. Her soul goes into the ancient past and shares the body of a girl from ancient times. The ML falls for the FL while his brother is in love with the girl whose body she shares. 

The ML and SML are half brothers who both fall for the FL.

The ML and SML are princes and half brothers who both fall for the FL.

ML's half brother lusts after the FL and the ML's brother-in-law and self-proclaimed but possibly deceived illegitimate half-brother both fall for the FL. 

The SML's stepmother unexpectedly turns out to be the ML's biological mother.

The ML and SML are not biological brothers, but the ML's father adopted the SML and SFL into the family to help look after the ML, who has a personality disorder. The ML and SML have fought since they were young and they both fall for the FL. 

Four of the princes (they're all brothers or half brothers) fall for the FL, and their dad also tries to marry her. 

Brothers

The ML, SML, and TML all fall for or pursue the FL to a certain extent. They are cousins and their grandfather forces them to live together with the FL.

The ML and his cousin.

One ML and his sister both fall for the other ML. The other ML obviously is not technically the FL, but his role in the narrative is that he is the one who is romantically pursued. 

Cousins

The FL was the SML's father's first love before she was frozen, and the SML's first love after she was frozen.