Is expiation (kaffara) obligatory for one who swore to hack his sister's Facebook account, but she doubted the reason for the oath, knowing that she does not know how to hack but intended to learn in order to fulfill her oath?

The intention affects the oath, specifying and generalizing it, and the reason takes its place when absent and indicates it. If it is highly probable that the reason for the oath was specific and that the sister did not do what was sworn upon, then no expiation is required. However, if the sister did what was sworn upon, then hacking or breaching the account is impermissible because it is forbidden, and in that case, an expiation for breaking an oath is required.