Does exchanging smartphones with my sister, such that she uses it for unlawful purposes, make me sinful, knowing that her previous use of the phone did not change after the exchange?

You bear no sin for what you mentioned because you were not the cause of her committing that disobedience. You should strive to advise your sister gently and kindly, reminding her of the seriousness of disobedience and its consequences in this world and the Hereafter.