The Twins Law was made many years ago because of a curse. The story goes like this: 

There were five original Werewolves but there must be four like the other original mythical creatures, they  have been cursed by the moon from their deeds after what they did to the original witch, the witch had called upon the moon because they had killed her sister in greed for power, The eldest werewolf, he is Salama King, and the second eldest is Rya King and the third eldest is the one and only Lucifer king. But the last two wolves, they're twins.  

The witch has cursed the twins. The two siblings, they were arguing over who would get the alpha spot the leader spot of all werewolves. The king.  They were named Gabel king and Reyna king A girl and a boy. Gabel was furious with his sister that she was a girl, meaning to become a boy. But Reyna wouldn't have it. Reyna argued that she was meant to be alive and not Gabel, because of his heartless ways and his plans that Reyna seems to know all the time. 

Werewolf twins were said that they could read their siblings minds to find out what they're thinking or planning, werewolf twins are rare. Reyna was the first she-wolf to ever be born. But ever since that curse, it was meant to be a challenge for the twins and only the Twins. Salama was confused on why he didn't get to become Alpha king, so he went to the witch and complained. But what the witch said is law, because they killed her sister in cold blood, she could never forgive them for what the moon goddess created. 

When Salama went to the witch, she said that the twins law is that they must challenge each other until death, and this law would continue forever. For those who has twin werewolves, must follow this law until broken. But to break it is to have the love from the other and not become a monster to the challenge of the Alpha.