My sister swore on Holy Quran when she was very angry not to use something which is very important for her. Later she regretted it; now she doesn't know what to do so she can use it again.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Since your sister wants to re-use something for which she has taken an oath not to use, then she has to expiate for this as mentioned in the Quran. Allah 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 Allah make clear to you His verses [i.e. revealed law] that you may be grateful.} [Quran 5:89]

‘Aa’ishah  said: “Unintentional oaths are those which occur by way of humour, argument or indefinite or unintentional or spontaneous talk, but the oaths which need an expatiation 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.”

Allaah knows best.