All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If your sister swore an oath not to use that thing and then wanted to use it, then she is obliged to expiate for breaking an oath, which is to feed ten poor persons, or clothing them, or freeing a slave. However, if she is unable to do any of these, then she should fast three days. Allaah Says (what means): {Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] — then a fast of three days [is required]. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allaah make clear to you His verses [i.e. revealed law] that you may be grateful.} [Quran 5:89].
Moreover, 'Aa'ishah said: ''Unintentional oaths are those which occur by way of humor, argument or indefinite or unintentional or spontaneous talk, but the oaths which need an expiation are the oaths which one makes by determination (while being serious), whether when angry or not, to do or not to do something, then Allah imposed an expiation for these kinds of oaths.'' [Az-Zuhri].
Allaah knows best.