My iPad is already associated with another account and I can't download any past purchases.

So back in April, I used my sister's account to download a book she had on her account on my iPad. I logged her out and logged back into my account on the iPad. I tried to download a book I had previously bought on my iPad but it says that I need to wait 90 days because my iPad is associated with another Apple ID. So while waiting for the 90 days to be up, I asked my sister to go on her Apple ID account to remove my iPad from her devices. It was removed and 90 days later, I tried to download previous purchases on my iPad only to see the same message pop up and tell me my iPad is still associated with an Apple ID and that I have to wait another 90 days.

What can I do to dissociated my sister's account with my iPad? Any solutions?