I know it's generally assumed to be Bernkestel but I don't think it's her either. Have you seen what she looks like in Umineko? She's a loli, not a flat-chested adult.
maximilianjenus said: They just got the character's body wrong in the anime.
Agree with that, they didn't know the proportions, and I suppose they didn't even know who Bernkastel was/is. So at the end of the anime that was only a mistake.