A while back I recall reading a fic where one of the Js marries or dates the other J's mother. The mother runs off, leaving her son behind (it's possible she passed in an accident, but I'm pretty sure she left of her own free will). It ends in J2... It was probably underage (that is: one character was under the age of 18), but I'm sure it wasn't extreme underage.

I'm sorry this is so vague. There are probably many stories with a premise similar to this. Please just recommend any you can think of. ♥