Ewen replaced Keisha Buchanan in the pop group last year, while original members Mutya Buena and Siobhan Donaghy have also previously left the girlband.
Ewen has now insisted that the current trio - consisting of Amelle Berrabah, Heidi Range and herself - is here to stay.
"The girls must be pretty tired now because it's the fourth lineup change," Ewen said. "Our management joked with us, if this doesn't work we'll have to call it quits."
The 24-year-old recently revealed that the group's new album will have an "organic" sound.