Why is a brother forbidden from marrying his sister?

The wisdom behind prohibiting marriage to close relatives such as mothers and sisters is to preserve dignity and safeguard honor. The Sharia has bestowed upon these relationships a reverence and respect that elevates them above the levity and playfulness that occurs between spouses. Sheikh Muhammad al-Tahir ibn Ashur pointed out that this prohibition aims to purify these relationships from the impurities of being used for amusement and pleasure, and that the sanctity of birth prevents any attempt at levity with the mother or the daughter. This sanctity extends to branches of kinship such as nieces (daughters of brothers and sisters), paternal aunts, and maternal aunts, so that the prohibition of marriage to them reverts to the principle of preserving dignity and safeguarding honor.