Wade, 31, and Union, 41, began dating in 2009, worked through a rough patch early this year and announced their engagement last week.
"ET" quoted an unnamed source as saying Wade has known the mother of his youngest son "for many years"; however, her identity has not been disclosed.
Wade was granted full custody of his other two sons after a custody battle with his ex-wife, Siohvaughn Funches. He is also raising his nephew, "ET" said.
Celebrity Families of 2014 [PHOTOS]