obtaining guardianship of minor in Texas

my aunt died leaving 2 adult children and a 12yr in Texas. The father of the 12yr never signed her birth certificate and does not want anything to do with her because she is the product of an extra-marital affair. The adult children have asked my husband and myself(their 1st cousin) to take guardianship because the are just 21 yrs old and can not provide adequate care. What are the proper steps to take in obtaining guardianship in the state of Texas. Can they just sign her over to us some kind of way. Please help!