Liam's father ran the mafia, and it was about time that Liam took over that role, helping him run it along with his four best friends. To step into the role they are given each guy is to take a wife, the girl usually has no say in the matter. If the guy chooses them then they are forced into the marriage.
Liam, being that he will be taking over as the one in charge gets to chose who he wants as his wife first. Followed by is right hand man, Harry. Followed by the others, Louis, Niall and Zayn.
Liam decides to prove his point of running a tight, trouble free mafia, by choosing the one girl who has caused the most trouble with everyone. She didn't want anything to do with the mafia, being forced into it by birth and has been trying to escape ever since. Liam decides to prove a point and get her and her two children under control.
Sorry my descriptions are always crappy, I promise you the story is better than the description I can write.