My son is a sociopath, but he seems to care for his sister?

My son was diagnosed as a sociopath 3 years ago, he has no empathy, feelings for anyone. He is 21 now and all he does is fight me and live in my basement. He finally moved out yesterday and my daughter who is 14 really does love him. They get along fine together and she always finds a way to get him to smile. When he was leaving she said to him " I love you big bro i love you more than anything else in this world you mean a lot to me" then she starting to cry, and he just stared at her and it was almost as if he felt something for her, his eyes got a little watery and he just smirked at her and left and said he will call her later. He called her last night and they talked for awhile, and he said he misses her and he cares about her. I dont know if he is lying but he always had a soft spot or whatever for her. He would protect her but doesnt 'love" her. is it possible that he cares for her at all or is just using her?