Sorry, but we do need to see the actual documents, so please post up what your sister has received, Having said that, the sum of £240 indicates that a Charge Certificate has been received as well as the PCN. (PCN =£160, plus 50% uplift for Charge Certificate = £80 giving £240 on the Charge Certificate.
Please also read this and update your thread as necessary. Only redact name and address from documents
As for the PCN, her options to pay or submit representations have not gone with the service of the Charge Certificate, (CC). There are two things she could do, other than pay the CC; (1) she could contact the council to explain what has happened, but this is outside the enforcement process, as there is no appeal option once the CC has been served, but she might be lucky. (2) ignore the CC and wait for the Order for Recovery to arrive. This adds £10 onto the £240, the small addition being the Traffic Enforcement Centre fee for registering the debt. At this stage she can submit a Statutory Declaration that the PCN was not received. OK, it eventually was received, but far outside the period allowed for payment or representations, so in reality was not received. TEC will then cancel the OfR, the CC, and the matter reverts to the PCN which you can then pay or appeal.