What are the consequences for me after I swore by Allah and by His Greatest Name (al-Ism al-Aʿẓam) not to forgive my sister, then it became clear to me that forgiveness is better?

Forgiving close relatives is a noble virtue and is commanded by Islamic law. Therefore, if you swore not to forgive your sister, you must forgive her and expiate your oath. The Prophet, peace and blessings be upon him, clarified that breaking an oath is preferable if doing so is better than upholding it, especially if upholding it would cause harm to family members. The expiation for an oath is to feed ten poor persons, or clothe them, or free a slave. If one cannot do that, then it is to fast for three days.