Complex sentence: MacMillan was the mother of Stefan Arngrim, who played Barry Lockridge on Land of The Giants (TV series), and Alison Arngrim, who played Nellie Oleson on "Little House on the Prairie".
Split into simpler sentences: MacMillan was the mother of Stefan Arngrim who played Barry Lockridge on Land of The Giants (TV series). And Alison Arngrim who played Nellie Oleson on "Little House on the Prairie".