In a way, yeah, Nancy Renoir was in a halfway point of turning Skullgirl.
There's some misinformation floating in some of the earlier posts in the thread so let's clear the air. Let's go to Double in Parasoul's story mode for closer details for Umbrella's affinity for Skullgirl power:
Double: As you suspected, as your mother carried your sister as she also carried the Skull Heart.
Double: Darling Umbrella is already attuned to the Skull Heart's power, thus she will forge a stronger and more terrifying bond than even that of your mother.
Parasoul: So... it's true. But I won't allow that to happen!
This is how I see the timeline based on info from the game and the
canon thread:
- -14 years: Skullgirl Selene Contiello defeated. Skull Heart's whereabouts unknown
- somewhere in the next four years: Double gives Skull Heart to Queen Nancy Renoir. She makes her wish and begins the slow transformation into a Skullgirl
- -10 years: Umbrella born
- -7 years: transformation complete! Skullgirl Nancy Renoir's rampage begins
When the Queen fully turned Skullgirl, Umbrella was 3 and Parasoul was 18. In the present, the two princesses are 10 and 25.
And now, Filia's story mode ending for insight about Skullgirl transformation:
Skull Heart: Your wish is pure... almost. Healing the broken girl is noble, yet you seek absolution for your forgotten guilt.
Skull Heart: Your transformation will be slow - make the most of it.
Filia is then shown in class, appearing to be normal, other than the occasional spike of pain as she slowly turns into a Skullgirl, which will probably complete in seven years (because, you know, there's this inexplicable 7 years Skullgirl pattern).