Sorry, but I am brand new to civil work. I have a witness who has given an affidavit, but is more than 150 miles away, so I can't subpoena her. She is a family member who is now under intense pressure from the rest of the family and is refusing to come to court voluntarily. Without her I have nothing. Is there any rule for unavailable witnesses in civil cases?

The only proper way you're going to get a substantive affidavit (as opposed to something like a business records affidavit) in is to use it as summary judgment evidence. It sounds like your best alternative is to either file a motion for summary judgment, or if you don't have the ammunition for that kind of move, look into deposing your reluctant witness, either orally or on written questions.

As you recognize, your witness is out of compulsory subpoena range, though some of my judges read Rule 176.3(a) to mean that if you subpoena the witness to appear in the witness's county of residence (e.g., you're willing to travel to depose them), they're obligated to appear and testify (I've been zapped by this interpretation with depositions on written questions before). The facial language of the rule could support that interpretation. Maybe it's worth a shot in your case.

If you haven't yet filed suit, you could file a petition in the county of the witness's residence for the court to issue an order authorizing her deposition for investigation of your claims under Rule 202.

This may not help but, as I understand it, the subpoena power is from the county line not county seat. Sometimes that can help.