דברי הימים א, פרק ג׳, פסוק ט׳

The conclusion of the royal genealogy draws a firm line between the different members of the king's household. The official record of the family refers strictly to the sons born to David's formal wives [מצודת דוד]. While these primary heirs are carefully named and counted, the children born to his concubines are intentionally left out of this detailed registry [ביאור שטיינזלץ].

The family record then makes a special note of Tamar, recognizing her as a sister [מצודת דוד]. Interestingly, she is not directly recorded as David's daughter. Instead, she is identified through her sibling relationship because she did not share a mother with everyone on the royal list. She is given the title of a sister specifically because she and Absalom were born to the exact same mother [ביאור שטיינזלץ].