Grete, Gregor's sister, may go through the most dramatic and genuine change in the entire novel. Whereas the father merely reverts back to adopting the roles that he is supposed to fulfill and did at an earlier point in the family's history, and the mother simply succumbs to her emotional weaknesses that were already present, the direction of Grete's psychological and emotional development is dramatically altered because of Gregor's transformation. Grete was on her way to growing up completely dependent upon her brother. His secret desire to send her to a conservatory to study violin may or may not have been her desire as well, but her devotion to her brother would likely have inspired her to follow his wishes especially since he was taking on the entire financial burden of her education. Grete is the only one to find the strength to directly deal with Gregor in his new form, and her newfound responsibility elevates her to a place of importance in the family, whereas she was just a child before.

Grete's self-appointed expert status on Gregor and his condition allows her to control the situation as best as anyone could. It is she that determines when the family has had enough and should rid themselves of the burden. Again, it is she that finds the most psychologically gentle way for the family to come to this decision by asserting that this creature is no longer her brother and so no one would ever fault any of the family for wishing him gone or attempting to rid themselves of the situation. Her revelation and decision making is what brings about the denouement of the novel and of the family.

It is at this point that Gregor's family has worked themselves into a strong unit consisting of self-sufficient individuals. Gregor's transformation allows every single one of them that their reliance on Gregor was never necessary and that...

The transformation woke them up to their sad existence, but his death made them realize that their existence wasn't nearly as bad as they insisted on believing it was. Gregor's psychological domination of the family was not intentional, and Gregor should not be faulted for the detrimental effect he had on his family, but what happened to him was ultimately necessary for his family to move on with their lives and to become valuable within their society.
One very interesting aspect of this situation is that all of Gregor's family members are free to feel this way because they have detached the memory of Gregor from the actual existence of this giant insect. They have essentially accepted that Gregor is dead, and that this creature that has replaced him is merely something to be tolerated until its eventual demise. Their distaste for the creature is more and more openly expressed as they psychologically disassociate their family member from what is left of him. They are all, of course, completely unaware of the fact that, though completely physically changed, Gregor still retains his mental capacity. It is this ignorance that allows them to treat the creature as vermin, not as a family member.

Gregor's transformation, though quite unfortunate for him, turns out to be a bit of a blessing in disguise for the rest of the family. He did all he could while he was able to care for his family, but it turns out that his kind of care was rather dysfunctional and ultimately only satisfied his own psychological needs. His being phased out of the family was the best thing to happen to any of them, and while the family is essentially better off without him, his ending is happy too because his family is happy, and he never has to worry about being late for work again.