[4:23] Forbidden to you are your mothers, and daughters, your sisters, your paternal aunts, and maternal aunts, your brother's daughters, your sister's daughters, your foster mothers who have given you milk, your foster sisters, your mothers-in-law, your step-daughters, who are in your care being born of your wives you have been in to - but if you have not yet been in to them you are not at fault - and the spouses of your sons who are of your loins, and that you should take to you two sisters together unless it be a thing of the past. God is ever Forgiving, Merciful.
To endeavor to find the intrinsic meanings behind the prohibitions is an impossible matter because the law does not need to be shown as having a cause (ghayr al-muʿallal). Instead, the Real (s) prohibits what He wills for whom He wills, and likewise gives permission. There is no underlying cause (ʿilla) for the laws at all: if the prohibited women were to be the permissible ones or the permissible women were to be the prohibited ones, it would be allowable.