was dewey always a brother or was he the next door neighbors son, in early episode they use to ask him to go home, need episode #

Dewey was always a brother. That's why the show is called Malcolm in the Middle, because he was the middle child. I just saw the first episode again on Nick at Nite a few nights ago, and he was definitely there as their brother.

You may be thinking of Christmas with the Kranks, in which Erik Per Sullivan (Dewey) played the son of the neighbor across the street that hassled the Kranks about Christmas.

Dewey was always one of the brothers. It is shown in the season 2 episode "Flashback" that he was born just after Malcolm accidently causes toxic fumes to fill the house. I'm not sure what you mean by them telling Dewey to go home, but I hope this helped!