Rose does want to take care of the ailing Flo, in her heart. She returns because it is her obligation. However, Flo is also the only mother figure she has known, and Rose may have had problems with her, but she feels family obligation and love for her.
In addition, the return to West Hanratty gives Rose a chance to re-examine her life at a time when things are .... complex. She has had a failed marriage, she has a daughter she can't seem to understand, a relationship with a married man, a love that is fatally ill. Rose has tripped about in her life, trying to find her identity, and it is only by returning home that she can gain some insight into herself and the roads she has chosen.