Quote 1
"You, Gregor!" cried his sister with raised fist and piercing eyes. These were the first words she had addressed directly to him since his metamorphosis. (2.26)
Finally, somebody says something to Gregor, but it's not a good thing. Grete throws his name at him as if it were an insult instead of a mark of his identity as her brother. This passage looks ahead to Part 3, where Grete refuses to call the bug "Gregor" and insists the family must find some way to dispose of it (see Quote #8 under "Family").
Quote 2
In the beginning she also used to call him over to her with words she probably considered friendly, like, "Come over here for a minute, you old dung beetle!" or "Look at that old dung beetle!" (3.8)
With the exception of Grete's calling out Gregor (see Quote #4), nobody calls Gregor by name again. The only person who says a word to Gregor is the cleaning woman, not any member of his family, and she calls him a "dung beetle." It's probably not as insulting if you actually are a vermin, and even though Gregor is annoyed, he detects the cleaning woman's "friendly" intent. It does make you wonder why the cleaning woman of all people should be the only character who makes such gestures to Gregor.
Quote 3
"I won't pronounce the name of my brother in front of this monster, and so all I say is: we have to try to get rid of it. We've done everything humanly possible to take care of it and to put up with it" (3.17)
By the time we get to this quote, the apple in Gregor's back is now covered with all kinds of trash (see Quote #7 above). Maybe Grete can forget her family duty so easily here because she can't see the apple anymore? In any case, she refuses to acknowledge the bug as her brother and calls it a "monster." The ease with which Grete can forget her family duty contributes to the story's generally scathing attitude toward family obligations.