Why was Gregor's sister not disgusted by him?
I think she is disgusted by him as demonstrated by the fact that she doesn't really look at him and always walks straight into the room and opens the window before she does anything else in the room. That said, she is more open to dealing with him because she has a close relationship with Gregor -- a sibling can be more of a friend than a parent can. The mother is so horrified at what has happened to her child that she just takes a lot longer to come to grips with it all. What mother can imagine such a horror for her own child? Gregor tells us that he has dreams for Grete going to the music school, so obviously they share a bond. Grete remembers his favorite foods and wants to try to please him by finding just the right foods for Gregor's new self.
There is also an element of Grete having a new and more important role in the family now. She is the strong caretaker who finds a new sense of self in her 'control' of Gregor's care. She derives some pleasure from this role, so she has to contain any disgust she has. It is interesting to see how this relationship changes by the end of chapter 2 and how by the end of the novella, she is the one no longer using even a personal pronoun to name Gregor, only calling him "it" and saying "it" has to go away.