Can you purchase directly from a T2? I really don't know. If you can I would give her written fair warning, make her sign a copy for your own records, that you will start purchasing books from HER account if she doesn't make the switch. make sure the warning states that, because you have tried several times to have the device removed from her account you are left with no choice but to purchase books through her account. If she tried to say you are stealing from her you can explain that she has the option of removing the device.
Other than that, I would keep pestering her until she deregisters just to get rid of you. But I'm a pain like that.
My son told me last night I won arguments with he and his sister because I was the most stubborn and they just gave up. If you make yourself enough of a nuisance she'll cave. She doesn't have any other option because she can't claim harassment or it will come out that she is withholding a vital function of a device she sold.