Joseph Fretti was discovered dead Sunday in his mother's Sarasota home, where he had taken his two young children for a visit, RadarOnline.com reported.
Fretti, the husband of Holmes' sister Tamera, owned a funeral home in the Holmes family's hometown of Toledo, Ohio.
Sarasota Medical Examiner Russell Vega told the entertainment news Web site the cause of Fretti's death hasn't been determined, but foul play or suicide aren't suspected.
A funeral mass is scheduled for Fretti at Gesu Church in Toledo Monday, RadarOnline.com said.
Holmes is best-known for her work on TV's "Dawson's Creek." She is married to Hollywood film star Tom Cruise.