She had two sons with her uncle.
The grandmother in question, is the mother of her mother, and the mother of her uncle/husband.
Such incestuous marriages (and the resulting high birth defect rate) are common in the islamic world. About half of marriages in Afghanistan are between first cousins.
Ali, whom the Shia (from the Arabic Shi atu 'Ali, "the partisans of Ali") believe was the rightful heir to Muhammad as Caliph, was Muhammad's cousin, nephew and son-in-law.
So they are destroying themselves? Nice!