In the trailer for Sylas we saw that in the past Demacia was prisoning the people with magic powers. But they didn't prison Lux, because she couldn't control her powers and she wasn't very confident. Maybe the demacian army general/commander/ something thought that they could manipulate her to use her as a weapon. Now, when Sylas is out of the prison he force Lux to choose between home and family, because Sylas is her brother.
So, i haven't read the 1st comic so that up there may be pure b*llsh*t, so please tell me if you have read it and i am wrong and write your ideas.