Complex sentence: The Texan officials began the treaty talks instead with demands that the Comanche considered impossible, including that the Comanche return all white prisoners, including people such as Cynthia Ann Parker, who were with bands of the Comanche not represented at the talks.
Split into simpler sentences: The Texan officials began the treaty talks instead with demands that the Comanche considered absolutely impossible, including that the Comanche return all white prisoners. This included people such as Cynthia Ann Parker, who were with bands of the Comanche not represented at the talks.