The fact that the spaniel instinctively slinks away means the following two things: first, he doesn’t like Dunsey; and second, the dog expects an argument to ensue whenever the two brothers are in a room together.
When one first reads the phrase “Godfrey’s face parted with some of its gloom…” one would expect the phrase to continue on to say that Godfrey’s face parted into a smile. Instead, the reader is surprised when they find out that Godfrey’s face actually parts into a frown. This is because one would expect grown siblings to get along with each other by this point in their lives. The fact that two siblings hate each other says there is something wrong with the family situation. In short, the sentence fools the reader into expecting the sentence to end a certain way, and it does not. 
The initial description of Dunsey paints him as an arrogant, irreverent malcontent. The phrase “heavy-looking” implies a dark quality in the character: heavy, as in burdened with faults or sins; heavy as in overweight; or heavy, as in carrying a dark secret. Although Eliot clearly portrays Dunsey as a malcontent, she still makes him seem a little mysterious by not letting the reader in to Dunsey’s full potential as a villain. After all, Dunstan is described as a deviant, but the reader is never keyed into the exact crimes that Dunstan has committed. 
Eliot also makes the character Dunsey seem mysterious by not telling us the motivations behind his tendency towards deviance. Was he born that way? Was he influenced by his father or brother? Was it caused by the death of his mother? Or had he just decided early in life that the lazy way is best? From the passage, one might ascertain that Dunsey realized early on that he already had what everyone else works for all their lives (money and a good reputation), so he had no real reason to have a work ethic. The way he addresses his brother implies a certain resignedness Can be reworded?a certain resignation to his fate:“I was obliged to come when you sent for me.” The first part of his utterance: “You’re my elders and betters…” implies that all his life, Dunsey was expected to be the mild younger brother who defers respectfully to everyone else. In a way, Dunsey does live up to this—he calls his brother “Master,” after all, but with a humorous twist. He comes when Godfrey summons him, but he makes sure to deliberately do so right after he has gotten drunk. On the surface Dunstan appears to be obedient, but he is also being subtly disobedient at the same time. Dunsey says, “I’ll do what you want, but only in my time and on my terms.” In a way, this is the worst form of disobedience. It is passive rebellion disguised as semi-obedience. In this way, Dunsey is almost controlling his older brother. 