The film's narrative relies on two major twists. First, Filumena's 'miraculous recovery' is revealed immediately after the wedding, exposing her 'illness' as a complete fabrication to entrap Domenico. Second, after the annulment is granted, Filumena plays her real ace: the existence of her three sons.
The ultimate resolution hinges on the mystery of paternity. Domenico spends the third act desperately trying to figure out which boy is his, looking for physical traits or instincts. Filumena steadfastly refuses to tell him, knowing that if he knew, he would reject the other two. The film ends with Domenico accepting all three as his sons, and they marry for real. The audience never learns which son is biologically his, reinforcing the theme that it doesn't matter.