What are the implications of a woman saying, "I will marry my son if my daughter returns home," after her daughter has, in fact, returned?

The statement issued by the woman is neither an oath nor does it entail expiation, because people are not accustomed to swearing by such words. However, if she said it as a vow to oblige herself to commit a sin, then expiation for an oath is incumbent upon her because a vow to commit a sin is not permissible to fulfill.