Is she sinful if she returns to using her sister’s belongings after having sworn not to and atoned for her oath, even though she feels obsessive doubts and avoids taking anything from her?

It is not permissible to supplicate against oneself, and an oath sworn by Allah is binding. However, breaking it is not forbidden unless the oath was to perform an obligation or avoid a prohibition. Breaking it may be permissible for the sake of maintaining kinship ties and preventing animosity. Expiation for an oath is permissible before breaking it. Expiation by fasting is not sufficient unless one is unable to feed or clothe the needy, or free a slave. Whoever performs the sworn-against act भूलकर (out of forgetfulness) is not required to offer expiation according to the majority of jurists. If the oath was expiated by fasting due to inability, no further expiation is required. However, if it was expiated by fasting while being able to perform other forms of expiation, the oath remains binding, and another expiation is required upon deliberately resuming the use of the items.