It is a song created by user JUSF周存 herself and published on BiliBili. I can ask her if she is allowed to use it in Lycoris radiata.
Oh,by the way,the story of this song is only the first chapter. The whole story has three chapters, and the remaining two are
《大小姐的逃亡生涯》（《The escape career of the princess》）
《大小姐的复仇生涯》（《The revenge career of the princess》），
the young master died and the princess
begins to revenge.