Is expiation incumbent upon one who swore not to speak to her sister for life, then regretted it and wishes to speak to her?

Expiation (kaffara) is required for breaking an oath. Whoever swears an oath concerning something and then sees that something else is better, must expiate for their oath and do what is good, as the Messenger of Allah, peace and blessings be upon him, said: "By Allah, if Allah wills, I will not swear an oath and then see something else as better than it, except that I will expiate for my oath and do that which is better."

However, if the oath was to do something forbidden or to neglect an obligation, then breaking it is obligatory. Based on this, you must expiate for your oath, speak with your sister, and uphold your ties of kinship.