Not exactly the same thing you're asking for, but hope some of these work.
Mirror of the Witch- The FL has a twin brother who is the crowned prince. They don't really switch places but they are both cursed before birth and her brother's curse is transferred to her when they are born.
Fabulous Boys/You're Beautiful/Ikemen Desu Ne- It was mentioned already but I just want to say that there's more than one version so you can watch them all.
Hwarang- The ML takes the place of the FL's brother and she believes he's her real brother.
My Girl- I'm currently watching this one. The FL is passing as the ML's long lost cousin because his grandpa was dying and his last wish was to find his granddaughter. After she's "found" he recovers so the FL has to keep playing the part of the cousin.
Rooftop Prince- The ML time traveled to the present from the Joseon era, he takes the place of his present re-incarnation who is currently in a coma after being attacked by his cousin who thinks he's dead.
She Was pretty- Not sisters but friends. The FL has her best friend take her place because she's self-conscious of how she looks and doesn't want the ML to see her (they are childhood friends that are reuniting and she sends her friend to meet him)
Ruler: Master of the Mask- The king (ML) is replaced by someone else (SML). He grew up wearing a mask so no one really knows how he looks.
I have not seen it but School 2015 has twin sisters switching places.
