So I am doing the same cosplay, I am drafting my own pattern which will probably be way out of your skill set, so I can't really help you there. BUT, I would definitely check on some Gothic Lolita patterns as well, the general dress shape, and the sleeves are pretty in line with gothic lolita dresses. The hardest part is going to be the front part where it changes from red to white as it is kind of a unique shape.
Sakura Con 2013 Cosplays
Lisbeth [Sword Art Online]
Fairy Kei Sei Fuku [Original Design]