How did JP's parents meet in the novel, A Face in Every Window?

Grandma Mary told JP that his parents met when they were children because Mary taught the ill Erin at home to protect her from germs. Mary, Patrick, and Erin built blanket tents and ate fudge. Having seen the movie The Sound of Music, Patrick decided that he should be the Captain and Erin his Maria. He offered her a bouquet of wildflowers and asked her to marry him. JP says that Grandma Mary told Patrick that marriage was different than how she and Patrick lived together. Nolan does not clarify if Patrick is JP's biological father, but she does offer hints that he is not when JP lists how he and Pap differ physically such as hair color and freckles