Is she actually being mistaken, or is she just taking the fall due to her onee-sama worship?
I think it's a reference to Operation Rheinübung. So she is getting mistaken with Bismarck like it has historically happened.
Schwester Bismarck! That's the Admiral's pudding!Who ate my pudding!?See, the admiral is upset!Admiral, you're wrong!!Oww...You're wrong! It was Schwester Bismarck!!You'll be scolded for that!!It was you, Puriketsu (Pudding Butt) Eugen! I won't forgive you for that today!Geez..I have to make something to eat, but there's nothing but sweets here!!AHH!!Huh?I'm sorry, Prinz...