Is it permissible to change the destination of a vowed charity (Eid sacrifice) to treat a sick sister, and does this necessitate expiation or delaying the vow?

The majority of scholars hold that a man's statement: "Upon me is the covenant of Allah," or "I have covenanted with Allah," or "I covenant with Allah" is an oath, and expiation becomes obligatory upon breaking it. This is based on the Almighty's saying: ﴿وَأَوْفُوا بِعَهْدِ اللَّهِ إِذَا عَاهَدْتُمْ وَلَا تَنْقُضُوا الْأَيْمَانَ بَعْدَ تَوْكِيدِهَا﴾ (And fulfill the covenant of Allah when you have covenanted, and do not break oaths after their confirmation), for Allah has called it an oath.

If you deem that spending the amount on your sister's treatment is more befitting, then spend it and offer an expiation for an oath, based on the Prophet's (peace and blessings be upon him) saying: "Whoever takes an oath and then sees something else as better, let him do that which is better and offer expiation for his oath."

Shaykh al-Islam Ibn Taymiyyah chose the view that these phrases take on the meaning of a vow (nadhr (definition)) if they involve committing to an act of worship, and they are an oath and a covenant if they are not. This is also the view chosen by Shaykh Ibn Uthaymeen, and it is the fatwa (definition) adopted on this website.

Accordingly, you have a choice between two matters: 1. Transferring the amount designated for the sacrifice to your sister's treatment, which is permissible and falls under transferring a vow to something better. 2. If you have not specified a particular year for your sacrifice, you can provide for your sister's treatment with the money you have available and delay the sacrifice until you can afford it, and there is no blame upon you.

In summary, bearing the expenses of your sister's treatment or maintaining ties with your father using your money is better than giving this money as charity or sacrificing with it. Therefore, it is permissible for you to spend the money from the vow on this better purpose.