One thing distinguishing Islam is the degree to which it not only encourages estate planning [as, for example, a military organization may encourage those about to enter combat (see http://GUNTRUST.ORG for a discussion of how estate planning relates to self-defense and combat mindset)], but also the great extent to which it actually regulates the planning itself in ways that seem contrary to many accepted norms of American society.
For a glimpse, check out an article on "Islamic Estate Planning" that was published June 2001 (interesting timing) in the California Lawyer. Since then I have used it a number of times to draft shariah-compliant estate plans for Muslim clients, and the article is now available online.
This area is tricky, may require difficult custom drafting, and you may find your clients consulting several imams for guidance as there may be disagreement on the correct approach. Nevertheless, the article serves as a good starting point. Here are a few excerpts:
Male and Female Shares
The Qur'an specifies that every male child gets the equivalent of two female shares (Surah an-Nisaa, "the women," 4, Ayah 11). This principle applies to males and females of the same degree of relation. However, an exception in 4:12 says that the uterine brother and uterine sister of the deceased (children of the same mother but not the same father) inherit equally, as do their descendants.
Spouses. A husband's share can be one-fourth to one-half share from the net estate. The wife or wives are entitled to one-eighth to one-fourth as a class. The spouse can get the maximum amount if the deceased leaves no descendants or (for the Sunnis) any agnatic descendants; otherwise they get the minimum.
Children. The Qur'an gives one daughter a one-half share, and multiple daughters split two-thirds. Sons are favored in all Islamic systems and convert the daughter's inheritance to a residual share in a proportion that gives the son twice what the daughter gets. Clients may also follow the Shia tradition of giving the father's Qur'an, ring, and sword to the eldest son.
The majority Sunnis say that a non-Muslim cannot inherit from a Muslim, or vice versa, but Shiites will allow a Muslim to inherit from non-Muslims.