Praise be to Allah.
Firstly:
If a Muslim swears to perform an act of obedience, it is recommended for them to fulfill their oath and not to break it.
Al-Mardawi (may Allah have mercy on him) said, "Fulfilling an oath regarding a recommended act is preferable." End quote from "Al-Insaf" (27/497).
Meaning: If the subject of the oath is a recommended act, then fulfilling the oath is preferable to breaking it.
If one breaks the oath and does not do what they swore to do, then they must offer expiation for the oath, by consensus of the Jurists.
Al-Wazir Ibn Hubayrah (may Allah have mercy on him) said, "They have unanimously agreed that a deliberate and binding oath is when one swears by Allah to do something in the future, whether to do it or not to do it, and if one breaks the oath, expiation becomes obligatory on them." End quote from "Hashiyah Ibn Al-Qasim `Ala Ar-Rawd Al-Murbi`" (7/469).
Secondly:
Allah the Almighty mentioned the expiation for an oath in the Quran: {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 Allāh make clear to you His verses [i.e., revealed law] that you may be grateful.} Surat Al-Ma'idah (5:89).
This has been explained in detail in the answer to question number (45676 ).
Thirdly:
If a Muslim offers expiation for their oath, then this expiation cancels out the oath, making it as if it never existed.
Al-Qurtubi said in his Tafsir (18/185), "Allah' saying (translation of the meaning): {Allah has already ordained for you [Muslims] the dissolution of your oaths} means the expiation of the oath is its dissolution."
Meaning: If you wish to make lawful what you have sworn upon." End quote.
Shaykh Al-Islam Ibn Taymiyah said, "{Allah has already ordained for you [Muslims] the dissolution of your oaths}... The meaning is: Allah has prescribed for you the dissolution of the oath, which is its annulment, the opposite of binding. And for this reason, those among our companions and others, like Abu Bakr `Abd Al-`Aziz, have used this verse as evidence for expiation before breaking the oath, because the dissolution does not occur after breaking the oath; for by breaking the oath, it is dissolved; and the dissolution only occurs if the expiation is given before breaking the oath, so that the oath may be dissolved, and after breaking the oath, it is only an expiation; because it expiates what is in the breaking of the oath from the cause of sin due to breaking Allah's covenant." End quote from Majmu` Al-Fatawa (35/253).
From this, it is understood that the dissolution of an oath occurs by one of two things: either by breaking the oath or by offering the expiation before breaking it.
Based on this, what is obligatory upon your sister is to offer the expiation for the oath, and by this, her oath will be dissolved, and nothing else is required of her.
And Allah knows best.
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