How can I get my niece to pay back money she owes us?
We cannot give legal advice, and are not forming an attorney-client relationship with you. The following is not a substitute for the advice of a local attorney. But we hope the information will be useful.
You have probably passed the time allowed by the statute of limitations on debt in Texas, which is 4 years. See: http://credit.about.com/od/statuteoflimitations/g/txsol.htm
If the statute of limitations (a time period after an offense during which you are allowed to sue someone) has expired, then your suit for the debt would probably be time-barred. A lawsuit would have been the way to pursue this. A mechanic's lien on a house is probably not. A mechanic's lien is for work done. This debt is not of that character.
It is worth talking to a local attorney who may have a different opinion.