Allah said,

﴿وَلاًّبَوَيْهِ لِكُلِّ وَحِدٍ مِّنْهُمَا السُّدُسُ﴾

(For parents, a sixth share of inheritance to each) There are several forms of the share that the parents get in the inheritance. 1. If the deceased left behind children, the parents get a sixth each. When the deceased had only one daughter, she gets half of the inheritance and the parents each one sixth, and another sixth is given to the father. 2. When the parents are the only inheritors, the mother gets one-third while the father gets the remaining two-thirds. In this case, the father's share will be twice the mother's share. If the deceased had a surviving spouse, the spouse gets half, in the case of a husband, or a fourth in the case of a surviving wife. In both cases, the mother of the deceased gets one-third of the remaining inheritance. This is because the remaining portion of the inheritance is treated just as the entire legacy in regard to the parents' share. Allah has given the mother one-half of what the father gets. Therefore, the mother gets a third of the remaining inheritance while the father gets two-thirds. 3. If the deceased left behind surviving brothers and sisters, whether half brothers, half sisters or from the same father and mother, their presence does not cause reduction in the father's share. Yet, their presence reduces the share of the mother to one-sixth instead of one-third, and the father gets the rest, when there are no other heirs. Ibn Abi Hatim recorded that Qatadah commented on the Ayah,

﴿فَإِن كَانَ لَهُ إِخْوَةٌ فَلاٌّمِّهِ السُّدُسُ﴾

(If the deceased left brothers or (sisters), the mother has a sixth.) "Their presence will reduce the share of the mother, but they will not inherit. If there is only one surviving brother, the mother's share will remain one-third, but her share will be reduced if there is more than one surviving brother. The people of knowledge attribute this reduction in the mother's share from one-third (to one-sixth) to the fact that the father is the one who helps the brothers (and sisters) of the deceased get married, spending from his own money for this purpose. The mother does not spend from her money for this purpose." This is a sound opinion.