Savy

This is...not good.

Lemme cook.

Shaman even said prophecies are vague and stuff so what if "fade into the shadows" means metaphorical death. The twin could be exiled or be removed from the family register/run away to another country.
I can see FLmc leaving to explore the world she's so curious about and living her best life. I just don't want this to end in tragedy. The MLmc likely will be the one who dies since A. He's got that illness trope b. His sister is physically stronger than him and this, in manwha, is usually a sign she's going to live.
And C she loves MLmc and would bow out to support him.