find my half-sister

My father, who I no longer have contact with had 3 childern last I knew. My half sister and brother are in New Mexico with their mother. Chelsea, my sister, sent me a letter years ago and the address got thrown away by our maid. I really want to contact her and tell her that I feel horrible for not replying and wished I could find her. I met her once in ‘99 when I spent a few days in California(where our father is). I also want to get in contact with my half brother Christopher because I believe he wrote part of the letter too. Hopefully one day I can find them.