I understand that you are referring to Chapters 3 and 4 where you see the majority of the references made to Dunstan.
From the incidents that occur, we see that Dunstan dominates is brother because of his conniving ways. He is idle, malicious, and holds over his brother a debt incurred by himself. He demands that his brother does as he says and pleases. He now put his brother in the awful predicament of selling his only horse in exchange for money they desperately need. Hence, what we get from the incidents is that Dunstan is not to be trusted, is mean, and disrespects those around him, whether it is family or not, and that he has the potential to really hurt someone when things do not go his way. He is heartless, and quite evil.