Thank you for your question. A step-parent adoption is legally permissible in Texas even if the parents are intending to divorce. Under Texas Family Code section 162.001(b):
A child residing in this state may be adopted if:
(1) the parent-child relationship as to each living parent of the child has been terminated or a suit for termination is joined with the suit for adoption;
(2) the parent whose rights have not been terminated is presently the spouse of the petitioner and the proceeding is for a stepparent adoption;
(3) the child is at least two years old, the parent-child relationship has been terminated with respect to one parent, the person seeking the adoption has been a managing conservator or has had actual care, possession, and control of the child for a period of six months preceding the adoption or is the child's former stepparent, and the nonterminated parent consents to the adoption; or
(4) the child is at least two years old, the parent-child relationship has been terminated with respect to one parent, and the person seeking the adoption is the child's former stepparent and has been a managing conservator or has had actual care, possession, and control of the child for a period of one year preceding the adoption.
So it would be permissible under the law. The challenge, however, is that the court has authority to deny an adoption permission if the adoption is not in the best interests of the child. Where an attorney might be reluctant is with regard to whether the adoption would actually be permitted, even if it is legally permissible. You could hypothetically petition the court for the adoption and the court could deny the petition based not on a prohibition but on the best interests of the child.
Obviously, every situation is different, so I have no way of assessing whether the girl you have raised would be better off if she was adopted even if you and her mother
are planning to divorce, but if the question is strictly whether an adoption is legally possible, when the adopting parent and legal parent are divorcing, the answer is "yes".
For reference, here is the full code section: http://www.statutes.legis.state.tx.us/Docs/FA/htm/FA.162.htm
Let me know if further clarification is needed, and once you are completely finished, please feel free to leave a positive rating (I work for ratings). Thanks.