Do i need power of attorney (bank)

Hello,

My sister has account in SBI. They have credited less interest due to mistake of one of their staff. They have accepted that mistake has been done from their side but they are not ready to give interest. I want to proceed this case to managerial level but my sister don't have time from her work. 

Can I represent my sister with Power of Attorney & handle further case.

If yes please suggest me the format of PoA.

Thanks in advance.