Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- It is only natural that God has made a female’s portion half of that of a man of the same degree of kinship whether she is an offspring or a full or paternal sibling.
2- These are the Islamic legal rulings. A Muslim must be advised to help his sister if she needs financial support and care. He must also be aware that this is a legal obligation upon him as long as his sister needs this sum.
3- These rulings pertaining to expenses or inheritance do not change whether or not a woman works.
4- There are cases where women receive equal shares as men or even more shares than men.
Answering your question, Dar Al-Ifta Al-Misriyyah, states:
Source of inheritance laws in Islam
Ijtihad
Some of the rulings on inheritance were established through ijtihad [independent legal reasoning] and these are subject to change according to interests.
Examples include a grandfather inheriting along with the deceased’s offspring, the inheritance of maternal relatives and so forth of other rulings established through ijtihad.
Textual evidence
Other rulings were enacted through explicit textual evidence from both the Quran and the mass transmitted Prophetic Sunnah [practice] and therefore are not subject to ijtihad.
Examples include the words of God the Almighty: {Allah instructs you concerning your children: for the male, what is equal to the share of two females.} (An-Nissa’ 4:11)
Whoever reads the verses on inheritance will notice that they clearly present the prescribed inheritance shares, the obligation to follow them, their reference to the wisdom behind the division, and that they were decreed by God the Most Wise Who places everything in its right place.
God the Almighty says: {Your parents or your children – you know not which of them are nearest to you in benefit. [These shares are] an obligation [imposed] by Allah. Indeed, Allah is ever knowing and Wise.} (An-Nisaa 4:11)
After mentioning the shares of parents, offspring (males and females), spouses and maternal siblings, the verses conclude by mentioning the promise of being admitted to paradise for those who respect the limits of God and a warning to those who disobey and surpass His limits.
God the Almighty says: {These are the limits [set by] Allah, and whoever obeys Allah and His Messenger will be admitted by Him to gardens [in Paradise] under which rivers flow, abiding eternally therein; and that is the great attainment. And whoever disobeys Allah and His Messenger and transgresses His limits – He will put him into the Fire to abide eternally therein, and he will have a humiliating punishment.} (An-Nisaa 4:13-14)
Eternal laws of inheritance in Islam
From the above Divine promise and warning and by designating inheritance divisions as God’s limits, we learn that the prescribed legislation of inheritance shares is eternal and cannot be changed or modified.
The rulings are ahkam ta’budiya [devotional rules] and a Muslim is obliged to follow them as they are. A Muslim must believe in their benefit both for himself and for his community and that God approved them for us until the Day of Judgment. Even though we believe in the fact that they are ahkam ta’abudiya, there is no harm in investigating the logical reasons behind them to understand their division.
Suitable shares
God the Almighty created men and women and distinguished each with a special composition and disposition. He legislated for each rulings that are suitable to their nature but gave them equal rights with respect to their humanity, freedom and dignity.
Furthermore, He charged each with the matters that are within their ability. God the Almighty says: “Allah imposes on no soul beyond its capacity.” (Al-Baqarah 2:286)
God gave men and women duties and rights that are suitable for the functions of each. He charged men with hard work and with providing for himself and his dependents.
Moreover, a man is obliged to financially support his wife even if she is wealthy for God the Almighty says: “Let a man of wealth spend from his wealth, and he whose provision is restricted – let him spend from what Allah has given him” (At-Talaq 65:7)
Why different shares in some cases between men and women?
A man is the guardian of his family and the provider of its material and moral needs. We cannot charge a woman with any of these obligations or burden with any financial duties. Furthermore, a woman is financially dependent on her husband if she marries. If not, she is either dependent on herself (if she is wealthy) or on her male guardian.
Based on this, it is only natural that God, the Most Wise, has made a female’s portion half of that of a man of the same degree of kinship whether she is an offspring or a full or paternal sibling.
These are the Islamic legal rulings. A Muslim must help his sister if she needs financial support and care. He must also be aware that this is a legal obligation upon him as long as his sister needs this sum.
These rulings pertaining to expenses or inheritance do not change whether or not a woman works.
Almighty Allah knows best.
Source: Dar al-Ifta al-Misriyyah