Why does my mother treat me so differently from my brother and sister?

Unfortunately, it sounds like your mom has some resentment about how you and your dad was closer rather then you and her. Sit down and talk to your mom about this and see what she has to say about this. Make an effort on your end to show her that you love her as well, like calling her, drop in on her unexpectedly, cook her a dinner. Most of all, let her know, maybe she has done this for so long that she is unaware of how you are feeling. Good Luck!