Mother-woman" is not so much a metaphor as it is a title and a statement of the role of women in Creole society. In Creole society at the turn of the century it was considered the ideal description of a woman. She is equally and indivisibly a mother to her children and a woman of her home and marriage. There is no separation of her roles, they are intertwined and this feels especially oppressive to Edna, while Adele is completely comfortable with it. Edna doesn't hate her children, but she wants to have her own life outside her home. She wants to enjoy the arts and learn to paint. She wants to learn to swim. All of these things have nothing to do with her children or her husband and home -- they are only for her. Another example that shows the difference is with music -- Adele enjoys fun songs that she can sing to entertain her family. Edna enjoys the classical music played by Reisz.
Adele serves as a foil character to Edna; she is the ideal mother-woman. Edna eventually defies every standard of that ideal, and realizes in the end that she can't truly live her life they way she wants, so she lets herself drown to escape it all.