Custody Issues, please help me.?

I apologize for publicizing this matter even though that is the purpose of this website. I am fifteen years old and I have been helping to raise my nephew that has been in my parents custody for the last two years. My parents work constantly so it was mainly just me and my nephew all the time and i loved it that way. I am obviously extremely attached to him and I become rather depressed when he is not around, he is my most important thing. However, his mother has suddenly cleaned her act up and has had him living with her for the passed three weeks and I have only seen him once since then. I am happy that my sister has more or less turned her life around and i want my nephew to have a mother... but i devastated me that he is gone. I hate to admit it but i am crying right now thinking about it (i never cry). My actual question is this: How awful of a person am I to wish he would still be with me instead of her? No matter how happy I should feel about the current situation I can't handle it! I wanted to take him to his first day of school two years from now and watch him grow up. I wanted to keep him for myself at both his and my sister expense and i feel guilty as hell about it. I know it's not right at all but i don't know what to do! Is it natural to feel this way or am I more shitty of a person than I thought? I could use some advise and criticism. Please answer.