Unfortunately, there is not much you can do. You can search for assistance programs to help you with rent options.
If your fiance's father is not willing to make repairs to the home, your only option really is to move out unless you have a legal lease. If you did, then you could take legal action, but do you really want to do that against family?
Like many in this economy, you are kind of stuck between a rock and a hard place. Ether move into another rental plae or apartment where living conditions might be better though costs may be much higher, or tough it out in your present home where rent is cheaper but living conditlions are not great. As I said, check out some of the assistance programs in the Fort Worth area and see if there is any assistance available to you that might help make your situation better.
Best of luck to you