Why does the share of brothers and sisters in verse 12 differ from their share in verse 176?

The first verse: (And if a man or a woman leaves neither parents nor children, and has a brother or a sister, then for each of them is a sixth; but if they are more than that, they are sharers in a third, after any bequest has been made or debt paid) [Quran 4:12]. The brothers and sisters referred to here are uterine siblings (from the mother's side). The share of their male and female is equal: a sixth for a single individual, and a third for a group, shared equally.

The second verse: (They ask you for a ruling. Say: Allah gives you a ruling concerning al-kalalah: If a man dies, and has no child, but leaves a sister, for her is half of what he leaves. And he inherits from her if she has no child. But if there are two sisters, for them are two-thirds of what he leaves. And if there are brothers and sisters, then for the male is the share of two females) [Quran 4:176]. The brothers and sisters meant here are agnatic siblings (full siblings or paternal siblings), and their inheritance is clarified in the verse.

The matter of inheritances is serious and complex, and it is not sufficient to rely merely on a fatwa. Rather, it must be referred to the Sharia courts for investigation and consideration of bequests, debts, and rights before the estate is divided.