If my sister would agree to power of attorney for her how would I go about getting the paperwork done for that?

If she is mentally challenged to the point that she could not enter into a contract to buy a car or lease an apartment, she lacks legal capacity to sign a Durable Power of Attorney (though she may have enough capacity to sign a Medical Power of Attorney, naming someone to speak for her when she cannot communicate).  You will then need to seek guardianship.  Your county probate court and the State Bar of Texas have lists of Approved Guardianship Attorneys.  Note that many practice in counties surrounding that in which they are registered and some practice statewide.