Auntie Marsh is Richard Marsh's younger sister and Kate Marsh's paternal aunt. In Episode 2, "Out of Time", she sent Kate a letter, in which she shamed Kate's actions in the viral video.
Letter from Auntie Marsh to Kate
Dear Katie
No spirit or devil could have prepared me for what you have done in that videotape.
And like a jezebel you released your harlot image before God's eye.
I will pray with my church for your soul to be saved from Eternal Hellfire.
Auntie Marsh