Castlevania: Portrait of Ruin

Did you kill whips memory (check you vampire killer, it should be stronger if you did)

sister's mode: get the good ending
Richiter mode: get the good ending and defeat the whip's memory boss

all you have to do is beat the game with the good ending ( get the sanctuary spell in the first desert painting and use it on the sisters then beat Brauner then beat Dracula ) then you start a new file on one of the 6 save files and it gives you the choices of hard or normal and Jonathan Richter or sisters

Yup. Beat the game with the best ending and make sure you beat the whips memory in order to get Richter and Maria mode. Use the sanctuary spell on the twins when they are sitting in the room before the final fight and then when the credits have gone up, start a new game and you will be able to choose between the sisters or richter for your next play through.