As I noted on the thread about the will calling for distribution “according to Islamic Laws and Sharia,” traditional Islamic law appears to call for sons to get double the share that daughters get. I should note, though, that Islamic law is in this respect better for women than is Old Testament law.
Numbers 27:8-11 provides,
Say to the Israelites, “If a man dies and leaves no son, give his inheritance to his daughter. If he has no daughter, give his inheritance to his brothers. If he has no brothers, give his inheritance to his father’s brothers. If his father had no brothers, give his inheritance to the nearest relative in his clan, that he may possess it. This is to have the force of law for the Israelites, as the LORD commanded Moses.”
This has been read as providing that daughters do not inherit at all if there are any living sons (though apparently they are entitled to some support from the estate, and to payment of wedding expenses). As best I can tell, some Orthodox Jews continue to endorse this rule.
Now, as I mentioned in my original post, I think (1) a person should be free to leave his property as he sees fit (subject to the duties to provide for a widow or widower, or for minor children while they are minors), whether he does for religious reasons or not, and (2) courts should generally not enforce provisions that call for the courts to apply religious law, at least in the many situations where the law may well be controversial given different modern religious views (e.g., “distribute my property according to Islamic Laws and Sharia [or Jewish Law]” would not be allowed, though “give my two sons 1/3 each and my two daughters 1/6 each [or give my son everything and my daughters nothing]” would be allowed). I don’t want to rehash these matters here. Also, I think (3) the better rule — the more morally sound rule, and more socially beneficial rule — is for people to treat their sons and daughters equally in such matters (though, as item 1 suggests, I certainly don’t think this should be a legally binding rule).
But here I just want to note that the Koran passages, as traditionally understood, are actually better for daughters than the Old Testament passages, as traditionally understood. To the extent that women are treated better today with regard to inheritance in many Christian and Jewish circles than in many Muslim circles, I think this stems not from the underlying scripture, but from the way that modern Christian and Jewish culture has generally evolved. That may be obvious to many readers, but in my experience many people claim that Islam is inherently worse in various ways than, say, Christianity or Judaism, because the Koran is inherently worse in various ways than the New Testament or the Old Testament or the combination of the two; I’m skeptical about such claims, and I’d be especially skeptical of such an explanation here.