I am married to the mother of my two daughters (and still legally married, but wife and I have been living separately in two states for over 5 years now).
First of all, if you want the most helpful responses, please answer the questions you were asked previously. Here they are, along with a few more:
1. Why are you uncertain about paternity?
2. You tagged your post as relating to Texas. Is that where you live? If so, where do your wife and daughters live? If they live in Texas, where do you live?
3. When did you get married (month and year)?
4. When did you separate (month and year)?
5. When were the children born (months and years)?
6. In what state(s) were the children born?
7. Do you have regular visitation with your children? If not, how do you expect to obtain genetic material?
8. Why have you allowed a separation to persist for five years without filing for divorce?
I have thoughts on consulting a family attorney with paternity and DNA testing, but should I approach an attorney in the state the kids are domiciled or the state I live in? Does location even matter?
Yes, it matters. However, the extent to which it matters depends on the relevant facts. When you come to a forum like this and ask for information and someone responds by asking you questions about the facts, you should answer those questions because they're important.