Why did the Incas marry their own sisters?

Incas married between brothers and sisters mostly to keep bloodlines pure. The Sampa Inca had to be (and was prefered) of pure Inca Blood (Inca was the nobility of the Tawantinsuyu which is known as inca empire). One of the most noticeable examples of this can be seen between brother Huascar (from pure Incan blood line) and Atahualpa (considered illegitimate or a "bastard" due to his mother not being from royal incan bloodline) and caused a civil war in the Incan empire.