You should hire an attorney even if the case is not being contested. Since your sister is their legal mother, she would have to either consent to the guardianship or rescind her rights to the daughter. Also, depending on their ages, it may be more appropriate to do a legal adoption rather than a guardianship. That is something you would want to take up with your attorney.
Generally speaking, in a guardianship in Texas, the court must appoint an Attorney Ad Litem to represent the interest of the proposed ward and may appoint a Guardian Ad Litem as well to make decisions about the proposed ward during the guardianship proceeding. Often, the Attorney Ad Litem and the Guardian Ad Litem will be the same person, an attorney trained in guardianship matters who is certified by the State to represent clients with special needs. Some courts may also appoint a court investigator who will represent the court in the matter, serving a role similar to the Attorney/Guardian Ad Litem.
The Ad Litem will meet the proposed ward, the applicant and any other relatives or persons in close contact with the proposed ward in order to make a determination as to the proposed ward's best interest. Once the Ad Litem has made his/her determination, a report will be written and filed with the Court. If the Ad Litem agrees that the proposed ward needs the guardianship and that the applicant is the right person for the job, then a simple prove-up hearing (where both the Applicant and proposed ward will be asked to answer questions for the record) will be scheduled with the Judge who will then make a final determination regarding the needs of the proposed ward. Only a Judge can legally remove a person's rights and give them to another person.
If the Applicant is appointed Guardian by the Court, he/she will have to take an oath, set a bond, and finalize guardianship documents which your attorney would prepare. The entire process can take a few months, mainly because the courts are busy and scheduling can be difficult with everybody involved, but once it is complete, you will be able to make important decisions for the well being of the ward. You should also be aware that the ward will be responsible for paying the fees of the Attorney/Guardian Ad Litem if the proposed ward has monetary assets available to them and is not listed as indigent. Your attorney can file an Indigent Petition which would allow the Attorney fees to be paid from the County Treasury.