Originally Posted by metroidreborn
In Revenge, LeChuck claims that Guybrush and he are brothers [...] Since Revenge, that whole brother concept was never mentioned again and was neither confirmed nor denied in the games.
Second, the fact that the skull works on LeChuck only proves that it's LeChuck's ancestor (of any relation), not Guybrush's father. How can Guybrush tell it's his parents just by looking at their skeletal remains? This isn't CSI. He says it is after like 1 second of looking at them.
Third, the "I am your brother" speech is an obvious rip-off of/reference to The Empire Strikes Back. It was most likely said for comedic value, not a plot device. It would be lame for the nature of Guybrush's eternal battle against LeChuck to hinge on a tired Star Wars reference, which is why the newer games don't pay attention to it.