Buthaina is a daughter of Al-Mutamid ibn Abad, but of Zaida this is not certain. According to Muslim scholars from Al-Andalus she was his daughter-in-law and not his daughter. Historians generally give more credit to those sources, and Fina Llorca Antolín confirmed that conclusion in "Las mujeres entre la realidad y la ficción: una mirada feminista a la literatura española" :

Levi-Provençal ha demostrado que no era hija, sino nuera, del rey de Sevilla, y no se sabe bien si llegó a casarse o no con el rey Alfonso VI.

in English: Levi-Provençal has shown that she was not the daughter but the daughter-in-law of the King of Seville, and it is undetermined whether or not she married King Alfonso VI.

[Fina Llorca Antolín (2008). Las mujeres entre la realidad y la ficción: una mirada feminista a la literatura española. Universidad de Granada. p. 91]

But, to be honest it must be said, that several writers still like to repeat that a different conclusion was drawn by some others in the past. It seems that most make use of the uncertainty to express their support of whatever their bias guides them to believe.