Is a woman sinful for not standing in line with other women for congregational prayer in the mosque?

You bear no sin for what you did, for straightening the rows is a Sunnah (definition) and not an obligation, especially since your action resulted from an error or forgetfulness, in accordance with the saying of the Prophet, peace and blessings be upon him: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are forced to do."