Confused & lost

Hello, I am very new to this site and am in desperate need for some advice. I have a younger half sister that was adopted. We have the same mom but different dads. She is now 20 years old, her family and my mom have always talked and kept in great contact. They send pictures every year, and we send them pictures of us as well. My sister knows that she was adopted from a early age, she has seen pictures of us too. We have never contacted directly. Well just the other day i was thinking about her, I wrote her mom a month ago and hadn't heard a response back. I thought to myself i wonder if she is on myspace, looked her up and sure enough there she was. I wrote her on her myspace, she ended up telling her mom that she did not want to talk to me yet and had them call my mom to tell me. I understand that it is hard for her but I still can't help from feeling hurt. She is my only sister and i guess a part of me was hoping that we could have that relationship. I have been told that she is not mad that she was adopted and that I am the only one who is upset. I guess i never really delt with it, and i have been mad at my mom for a long time. I don't know if by feeling that way I am being selfish maybe. Well she still has her account, she just put it to private so i can no longer see her pictures or her life. I went ahead and kept my account open in the hopes that one day she will write. I am confused that if she is not upset then why she would not want to talk to me? I can't quit thinking about it and i am feeling real lost and confused. Any advice would be greatly helpful, as i don't know anyone else that has gone through this. Thanks so much.