What is the ruling on one who swore to forbid her prayer and fasting upon herself if her daughter went to a relative's wedding?

If you did not swear an oath but rather conditioned the prohibition of prayer and fasting on your daughter going to a specific place, then no expiation for an oath is binding upon you, whether your daughter goes or not. It is also not permissible for you to abandon prayer and fasting. You must guard your tongue and repent to Allah Almighty, and busy your tongue with the remembrance of Allah, the recitation of the Quran, and seeking forgiveness.