Gregor always loved his sister and her paying of the violin. Before his "condition," he had thought seriously of saving up money so that he could send her to the conservatory. Later, in his changed physical state, he had fewer and fewer pleasures, but his sister's playing was definitley one his very favorite things. As she played for the lodgers,:
Gregor crept forward still a little further and kept his head close against the floor in order to be able to catch her gaze if possible. Was he an animal that music so seized him? For him it was as if the way to the unknown nourishment he craved was revealing itself to him.
Nothing seemed to matter to Gregor any longer but the nearness of his sister and the beauty and tenderness of her playing. In all his obvious, animal/insect repulsiveness there was still a sensitive, responsive and loving human heart.
But from this point on, Grete begins to vehemently reject her transformed brother, as if she has been finally and totally disgusted by his presence and his desire to be near her.
Gregor reacted strongly to Grete's violin playing. When Grete was playing the violin the boarders came to listen, as did Gregor. This did not end well, as the boarders were shocked to say the least and stated that they would not pay. Eventually Gregor's father kicks them out. This section tells us three things about Gregor.
First, Gregor wonders whether his reaction to the music was the result of his humanity or his bestiality. In this sense, Gregor is confused.
Second, he still loves Grete and his family. In fact, he imagines in his mind how he will tell Grete about his love of her playing, and how he wants to send her to a conservatory to hone her skills further. He cannot, of course, as he is a bug.
Third, there is still enough of his humanity left for him to desire to take care of his family.
All of this shows that Gregor is not completely an insect. He still has traces of his humanity.
Finally, this occurs ironically when his family cannot take him any more. They plan their lives without him.