Signing a PiP AR1 (Review) form

Hi everyone, firstly thank you for the outstanding service you provide - I have been a member for the past 8 years and used your guides for PiP, ESA and UC, thank you! I have a question which I can't seem to find the answer to regarding Section 1 of the PiP AR1 review form - "Read and Sign the Declaration". I am the attorney for my mother (we have a LPA registered with the OPG). I will complete the form on behalf of my mother but I am unsure if I sign section 1 or if my mother is required to sign it (the AR1 does not provide a space asking if the form is being completed on behalf of the claimant - unlike in the initial claim -). Does anyone perhaps know who should sign the form - I don't want the form rejected due to the incorrect signatory! (ps the AR1 was posted to me the attorney by the DWP if that helps!)? Many thanks