Norb does care about Dag, it's just that I think that the show was trying to show the good and bad side of their sibling rivalry. The first two seasons, Season 1 and Season 2, showed the good side of their relationship, meaning that Norb and Dag would fight each other, but they always make up with each other at the end of the episode. However, in the later seasons, Season 3 and Season 4, those two seasons were showing the bad side of their relationship as they barely got along with each other. Norb wasn't always portrayed as being very manipulative towards Dag, it's just that the show went through "flanderization." Here is a brief definition of what "flanderization" means:
"The act of taking a single (often minor) action of a character or event within a work and exaggerating it more and more until it's huge and cartoonish and almost entirely consumes the character or work in question." From TV tropes.com
In other words, the show ended up exaggerating Dag's stupidity and Norb's teasing of Dag in the later seasons, which is why their personalities were so different in the later seasons, same thing that happened to the later episodes of "Spongebob Squarepants." Point is, the earlier seasons showed how much Norb and Dag cared for each other, while the later seasons showed how much they couldn't stand each other (or more like how Norb can't stand Dag). I think that the show had to take a different direction in order to show the bad side of their relationship because if the show had stuck with the original formula, then we would have seen the same routine between Norb and Dag over and over again. So, yeah Norb cares for Dag just as Dag cares for Norb, it's just that when the show wanted to show the bad side of their relationship, they had to make Norb seem more manipulative in the later seasons in order to pull this off, which is why it was so difficult to tell if Norb actually cares for Dag. Besides, if Norb actually hated Dag, then why is he still living with Dag? Norb could have easily left Dag alone if he wanted to. He must have some love for him just to stay with Dag for a long time. Again, I think it was hard to tell if Norb cares about Dag because of the excessive "flanderizing" of the characters' personalities.
Also, here's a list of episodes where Norb cares about Dag:
1) Fakin' It - Norb was very worry about Dag being sick.
2) Tree's Company - Norb was upset when Dag left the dam.
3) Lumberjack's Delight - Norb was angry when the lumberjacks did something to Dag... I forgot...
4) Vantastic Voyage - Norb was definitely worried about Dag and even tried to save his life.
5) Yak in the Sack - Norb saves Dag from the Yak.
6) 3 Dag Nite - Norb exclaims when he is out of the dam: "But I'll never find another Dag."
7) I Dare You - Norb tries to save Dag by telling him that he will munch on the wood.
Shell or High Water - Norb tells Dag that he loves him after Dag confesses that he loves Norb.
9) Damnesia - Norb was very happy to know that Dag loves him and was concerned when Dag hit his head.