All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Muhammad and upon all of his family and companions.

The vow taken to perform an act of obedience for The Sake of Allah has to be fulfilled as He Enjoined and Said (What means): ” Let them perform their vows.” [Al-Hajj/29];in addition,  Allah, The Exalted, Praises those who fulfill their vows and Said (What means):” They perform (their) vows, and they fear a Day whose evil flies far and wide.” [Al-Insan/7].

Nevertheless, you (Questioner) won’t be free from the liability so long as you have money and obliged to pay the aforementioned part to your sister in order to fulfill the oath you took as much as possible.

In this regard, Al-Imam An-Nawawi (May Allah have mercy on his soul) said (What means): ”The vow taken to perform an act of obedience is to be considered and fulfilled regardless of what a person linked it with save if it was for a prohibited purpose.” [Al-Majmoo’ vol.8 pp.458].

So if you couldn’t carry on fulfilling your vow at some point; some jurists permitted offering an oath expiation instead, as is the case with the Hanbali’s school of thought in this regard.

Moreover, Ibn Qudama (May Allah have mercy on his soul) said: ”Whoever took a vow to perform an act of obedience in which he couldn’t afford or unable to fulfill, the he is obliged to offer oath expiation in this case for  It was narrated that 'Uqbah bin 'Amir said:"My sister vowed to walk to the House of Allah, and she told me to ask the Messenger of Allah about that. So I asked the Prophet for her and he said: 'Let her walk, and let her ride." [Agreed upon]. A different narration for Abi Dawood: ”Let her offer oath expiation.”. Further Ibn Abbas (May Allah be pleased with them) said :”Whoever took a vow in which he couldn’t afford is obliged to offer the same expiation of an oath.” [Al-Mughni vol.10 pp.11]. And Allah knows best.