In the particular cases where the Quran gives a son twice a daughter's share, the difference is tied to a corresponding financial burden: the man must pay the mahr and bear the full maintenance of his wife and children, whereas whatever a woman inherits is hers alone and she is obliged to spend nothing of it on anyone (Quran 4:11; 4:4; 4:34; 2:233). The rule is also not universal — siblings through the mother inherit equally regardless of gender, each taking a sixth, or sharing a third if they are more than two (Quran 4:12), and a mother and father each take a sixth when the deceased leaves a child (Quran 4:11) — so there are cases where a woman receives the same as a man, and cases where she receives more than some male heirs. These fixed shares are revealed law: Allah calls them "an obligation from Allah" at the close of Quran 4:11 and immediately follows the inheritance verses with the warning against transgressing His limits (Quran 4:13-14). So they may not be altered by custom, by pressure from relatives, or by a will — a bequest may not be made to an heir in order to redistribute what Allah has already apportioned.
Q&A · Women in Islam
Why does a woman inherit half of what a man inherits?
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.