I didn't find the quote I was looking for, but I found this for anyone that's interested. Thought it might be relevant to episode 7.
K: Thanks. I think the next question is the key to EP4, that is, the dark side of Maria's magic. This is a very unexpected element. With respect to the plot up to this point, this is also a very important addition. Is this something you've planned on revealing in EP4 from the beginning?
R: This is one of the themes we'd planned on revealing in the question arc. You can think of it as a major clue to revealing the world view... eh, it's difficult to describe.
K: Is it because this question is too close to the core of the story?
R: If you fully understand the world of Umineko, then you would find that this is an additional element pertaining to Beatrice. Without the meeting with Maria, there wouldn't be this present Beatrice as she is now. To Beatrice, Maria is an existence she cannot belittle; moreover, she is a crucial element in creating the "present Beatrice". So just talking about the plot of the story alone, the meeting between Maria and Beatrice at least causes some chemical reaction to the world of 19
EDIT: Okay I gave up on looking for that quote, but here is the list of interviews I know of.
interview #'s 1-3
(split into two parts)
France Convention interview