Not making weight and violating a contract is one thing.
Having a blood related problem is another.

In short, no Mayorga can't sue Vargas.

As far as fighters suing fighters, theres been a lot.

The one that comes to mind is Joey Gamache suing Gatti I think, or Gamache was suing the States A.C. for allowing Gatti to fight him despite Gatti weighing a lot more then what was allowed come fight night. I think Gatti weighed something like 165 lbs. on the day of the fight. The fight was being held at 140 lbs. Something happened the day of the same-day weigh in I think while Gatti was being weighed they did something to hide the fact that he had blown up in weight and had gone past the allowed weight that could be gained.

If I'm not mistaken this law suit just settled not too long ago.