That is a Hymenocallis not an Ismene. The corona is far too big for H. harrisoniana as well as H. maximiliani. H. palmeri would have a different form to the corona evident even at this age of flower and the tepals would be chartruese. There are a number of possible species and hybrids it could be. It won't be possible to identify entirely without more details of the plant and possibly where it is from though it could be narrowed down to less choices. -- Alani