@chaos86 if I'm not wrong, earlier chapters already described her motive.
Simply put, her family is too strong in terms of influence, stronger than the royal family perhaps. Yet the connection is simply too fragile since her father is too trustful with his connections. Once the royal family decided to crush the Alberts (her family), literally no one will take their side, causing an ultimate ruin
In other to save the family, Mary choose to "sacrifice" herself through the condemnation (typical of otomege as you know). By doing this, the only one who falls into ruin will be Mary alone, not the entire family