Well, that depends on the word "brother". Since all Kaiju are clones of each other, they aren't really any sort of family. Knifehead may possess some of Tresspassers more "unique" genetic traits, making it sort of a Tresspasser Mark 2, an improvement over the original, but they aren't quite "brothers"
It should also be noted that the similarity between their names is simply because those were their codenames, given to them by humans. We have no idea what, if any, names Kaiju possess.