Who are those 2 Latino twins? Why did they murder all those Mexican immigrants in the truck?

Those two are cousins; relatives of Tuco. They were sneaking into the US from Mexico for the sole purpose of tracking down and killing the person responsible for Tuco's death. They killed the immigrants and truck driver to avoid having witnesses that could identify them.

The two latino twins are Tucos cousins they murdered all the immigrants because that boy that was talking to them noticed their boots and the silver skull on the tip of them and that was a sign that they were part of a a Mexican cartel that's why after looking at the boots he stopped talking and looked away. The twins noticed this and knew that he had realized they were from a Mexican cartel so they killed him along with all the rest in the truck

They are Tuco's cousins and they've come to get revenge on Walter White. They kill the immigrants because dead men tell no tales.