ok this might sound a bit crazy so let me try and explain.
Lady Gisela could easily be Sophies mom (whhhhyyyy)
Shows interest in sophie
likes messing with genetics (gorgadon)
Poyglot
Photographic memory
blonde
Sort of manipulates Keefe into spying on her
but Mr. Forkle says they didnt know each other, so that pretty much rules lord Cussius out. So what if it were Counciller Bronte??? Mr. Forkle said the elvin world could "topple" and councillers can't have familes so it makes sense.
Another thing: Why does Silveny like Keefe and Sophie?? What if Silveny some how can sense the bond between them and that makes her like the pair of possible siblings.
Also Keefe has a Photographic memory too