Is it obligatory for me to attend my sister's wedding after I swore strong oaths not to attend, invoking upon myself entry into Hell in the event of breaking the oath, and is expiation sufficient for me for breaking the oath in this situation?

Attend your sister's wedding, expiate your oath, and repent to Allah. Do not resort to such oaths and supplications again, for Allah the Almighty says: (And do not make [your oath by] Allah an obstacle against your doing good, or fearing Allah , or making peace among people. And Allah is Hearing and Knowing). The Prophet, peace and blessings be upon him, legislated expiation for an oath if it was sworn to commit a forbidden act or to abandon acts of obedience and duties. On the authority of Abu Hurayrah, may Allah be pleased with him, he said: The Messenger of Allah, peace and blessings be upon him, said: (Whoever swears an oath and then sees something better than it, let him do what is better and expiate his oath). As for your supplication, do not let it be an impediment to reviewing the truth, and do not fear its consequence, for Allah the Almighty says: (And if Allah were to hasten for the people the evil [they invoke] as He hastens for them the good, their term would have been concluded for them. But We leave those who do not expect Our meeting to wander blindly in their transgression.)