Communication with a third party

What an incompetence at standard bank vehicle and asset finance department. We made an agreement that the installments of a car bought under my name be deducted from my sister's account, they have an agreement letter and her proof of banking details as lwell as her contact details. They deduct the installments from her account but they refuse to communicate with her when the need arises. They call her a third party yet they collect money from the third party. They refuse to make arrangements for arrears in the account with her yet they always preach we are committed to help you. Every consultant says whatever she wants to say. A power of attorney letter has also been sent to *** and nobody is attending to it yet they said they want it in order to communicate with her. This is a bit dissapointing.