Er...crafting?
Oh oh, using your reservoir. Hee, okay then...
That'd be tricky...but maybe not out of the question. Let me summarize the background as it stands right now, with the understanding that it's still "soft" as it were, so we can still make changes without messing things up.
Sigil's mother lived in a small town, where she was viewed as scandalous because of her strange ways (druidic faith, perhaps) and her unmarried status. The scandal intensified when she became pregnant with what turned out to be a lovely baby girl, and the father was unknown.
At this point, perhaps Sigil's mom and Katherine's dad met somehow. It may be that her mom got a reputation as a healer and 'wise woman' of sorts, and Kat's dad...a local ruler...had need of such counsel. Maybe he had an illness that was of a somewhat compromising nature, so he didn't want the clergy to know about it. Or maybe they just crossed paths and her beauty smote him or something.
Then the girls grew up together for awhile in the baron's keep. Sigil (not what she was called then, of course) had a powerful draw to the woods and lakes and loved to explore them, to the dismay of caretakers. Kat was probably assigned to go get her a lot, and keep an eye on her. I think there might have been some inkling of her abilities...perhaps she liked talking to the castle animals, and they seemed to actually listen, or she saw things that didn't seem to be there... She may have seemed "slow" or "special."
And then one day she vanished. For a week or so before, she'd been especially scarce and seemed distracted. More so than usual. There were strange lights and sounds coming from her room, though never odd enough to get adult attention. She would have been in her early teens, perhaps... Then one night she sneaked out, and never came back. Dogs tracked her into the woods, but her tracks ended abruptly in a little circular clearing ringed by old swollen toadstools.
As time raced by in the mortal world, Sigil was embroiled in a power struggle in Faerie as the current ruler of what was at least a large swath of that world was a feared and despised tyrant known only as the Winter Queen. She joined a tiny underground resistance and began unlocking the mysteries of her blood as she fought the armies of Winter. In the big final confrontation Sigil learned that the Winter Queen herself was also of mixed mortal and fey blood, and that she had first opposed a terrible warrior queen when she'd arrived. In short, she learned that this was but another chapter in a book written from blood, and in vanquishing Winter and becoming the next Queen of Summer, she wasn't ending happily ever after, but embarking on a path to her own eventual corruption and destruction.
I'm thinking at this point, maybe Sigil tried to return to the mortal world, but found it in the grip of this warlord's reign, and so fled back to Faerie. Whether or not that happened, she then stayed in Faerie for long ages, but eventually realized that the evergreen forests were starting to turn orange and yellow, one leaf at a time. The grass was ever so slowly yellowing and ripening. And her heart was gradually turning harder, and colder. Things seemed less fun now. Her patience wore thin faster. More and more, she had moments where she felt like she was surrounded by cretins...that the only way they'd be worth anything would be if they were -forced- to be worth something.
Sigil realized that the cycle couldn't be broken...winter was coming, and it would arrive. But that didn't mean SHE had to be the Winter Queen. She had human blood as well as fey. The bond that let her cross one way should, in theory, let her cross back. So she took her most loyal servant, and her 'mobile home,' used for long treks to distant corners of the realm, and took them to the crossroads when the stars were right. There she abdicated the throne and title of Summer Queen, along with the powers and privileges therof, which would (presumably, no one really knew) open it up for the fey to fill with another candidate, and used the crossroads to pass back into the mortal world.
Her return to the mortal world can be as long or as short a time ago as is needed for background and plot elements. I do think it shouldn't have been TOO long, since one thing she would do fairly quickly on getting back is visit the tomb of the Warlord, who's bloody ascension to power she had witnessed before crossing the veil to Faerie in the first place. It is one of the few things that ties the world of the present to what she remembers.
Anything we can work with in that?