My cousin passed away recently from hyper eosinophilic syndrome. what causes this?

Multiple causes: It is a condition characterized by too many eosinophils (a type of immune cell). Sometimes a mutation in the precursors of the eosinophils causes them to multiply out of control. In other cases, another immune cell goes haywire and tells the eosinophils to multiply. Sometimes we can't find what causing them to multiply. It is usually acquired through life, and is only very, very rarely inherited.