Responses
Seungho didn't receive any letter from his brother.
He ~supposedly~ received a letter from a Lord Song that is a character that we don't know yet but apparently it's someone related to Seungho's past during the time his father locked him.
Jihwa somehow is aware of Lord Song and he used this information to lure Seungho by writing a letter pretending to be him. Seungho recognized Jihwa's calligraphy.
Omg thank you so much for the clarification!
The letter Seungho recieved from his brother was actually written by Jihwa?
(・・;)ゞ