Just ask yourself why a loving mother would abandon her newborn daughter and her husband. 1) She's an assassin and needs to disappear after a botched job 2) Her family was promised in a government experiment so she left them to allow that experiment to happen 3) She was abducted by aliens or is an alien or THEY were abducted by aliens 4) She was abducted by human sex traffickers, smuggled into a foreign country, and is now drugged to the extent that she hardly knows her own name 5) She's dead..killed herself/had premonition she would die/sacrificed herself, etc. 6) She has fugue (a condition where someone has temporary amnesia and they assume an entirely different identity and move away and start a brand new life, forgetting they had a life prior to this) 7) The child was born with a rare disease and the mother was the donor...but no one knows because it's a controversial and illegal procedure (like human cloning). Etc.
The possibilities are endless. Ask yourself what you want to accomplish with the story. What the theme is.
What interested you most about that story. Try to pin point what the point of the story is and that should help you develop the plot details more. Good luck.
If this is a fantasy novel, than you can have the mother/wife leaving because she has missed another world that she discovered as a kid and she wanted to go back, but couldn't find her way back due to a villian or something else.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.