In Matthew 10:3 we have this: Philip, and Bartholomew; Thomas, and Matthew the publican; James the son of Alphaeus, and Lebbaeus, whose surname was Thaddaeus;
In Mark 3:18 we have this: And Andrew, and Philip, and Bartholomew, and Matthew, and Thomas, and James the son of Alphaeus, and Thaddaeus, and Simon the Canaanite,
We notice two things: (1) in both verses "the son" is in italics, which means that it is not in the Greek text. In Mt 10:3 it says James "of the" Alphaeus, and in Mk 3:18 it says the same thing. So who was James "of the" Alphaeus? It could only mean "son of". (2) But Judas is called "Lebbaeus" and "Thaddeus". However in Luke 6:16 we have And Judas the brother of James, and Judas Iscariot, which also was the traitor.
So we see that (a) Judas is also Lebbaeus or "Thaddeus" and (b) he is the brother of James the son of Alphaeus.