Ahh I see, I read the spoilers below (thanks guys!) so in summary:
Black-hair husband = trash Was sleeping with the mistress because he's not allowed to touch FL because she's sick. Also because he was commanded by the 1st Prince to give her to him later.
Blonde-hair 2nd Prince = ML Fell in love with FL and danced with her when they were young. Sent to war so that he can't have FL.
1st Prince = Bad Guy Secretly commanded Black-haired husband to marry FL in the first place, poison her, then take her corpse like a doll.
Mistress = FL's Twin Sister Jealous of FL's life. So took her husband and became his mistress. But then (might have) felt bad for FL when she knew she was sick, and so gave her body to her with magic.
.
.
.
.
.
.
.
Ahh I see, I read the spoilers below (thanks guys!) so in summary:
Black-hair husband = trash
Was sleeping with the mistress because he's not allowed to touch FL because she's sick. Also because he was commanded by the 1st Prince to give her to him later.
Blonde-hair 2nd Prince = ML
Fell in love with FL and danced with her when they were young. Sent to war so that he can't have FL.
1st Prince = Bad Guy
Secretly commanded Black-haired husband to marry FL in the first place, poison her, then take her corpse like a doll.
Mistress = FL's Twin Sister
Jealous of FL's life. So took her husband and became his mistress. But then (might have) felt bad for FL when she knew she was sick, and so gave her body to her with magic.