What are some methods to find someone who doesn't want to be found?

I went to my local PD and asked them if they could try to find my sister and do a wellness check on her and her daughters because she is married to a very abusive person and we haven't seen her or the girls in 6 months. They said they would have to have the address, if I had the address I wouldn't have needed to go there! I know that she is recieving foodstamps and medical from the State for the girls and I told them at the PD that and just got a blank stare back. All I want to do is see the girls and make sure thay are well! Any legal ideas? We live in Texas.