Is expiation obligatory for one who swore not to tell anyone, then told? And is expiation obligatory upon my sister for her false oaths and stealing when she was nine years old?

You are required to offer expiation for breaking an oath because you violated your oath not to inform anyone about what your sister said. The expiation is: feeding ten poor persons, or clothing them, or emancipating a slave. If you cannot find any of these, then you must fast for three days. There is no sin or expiation required from the young girl who has not reached puberty, due to the Prophet's (peace be upon him) saying: "The pen has been lifted from three," and he mentioned among them: "the child until he grows up."