Any ideas on how I can locate my neice & nephew?

[Annie]

My older half sister is basically MIA (drugs). She has children and I would like to get ahold of them. I talked to her aunt today who said the family does not talk to her or her children, and isn't sure where they are. She knows that the oldest boy (who is 19) lives around there because she has run into him, and the oldest girl (who is 17) lives with the rest of her siblings with someone one of my sister's boyfriends relatives. I tried myspace and facebook...any other ideas? Thanks!