A proper contingency plan would specify suitable people to stand in as a deputy clerk, or measures that would need to be followed to identify such people and engage them. The plan would need to be approved by the whole council, particularly if the clerk is also the responsible financial officer. In theory the Clerk's daughter may be able to fulfil the role but only if the Council can determine what criteria she would need to meet,
GDPR breach depends what "showing" her daughter involves. On the abscence of demonstrable compliance being established, if showing involves her daughter processing any paritioner data or correspondence with someones personal data on it, that could be a breach.
Making copies of it, extracting it and storing copies on a non council PC, storage or network drive would be a breach.