12. " and you will have half of what your wives leave, if they have no child.
but if they have a child, then you shall have a fourth of what they
leave after (the payment of) any bequest they may have bequeathed or a debt;
and they shall have a fourth of what you leave if you have no child,
but if you have a child, then they shall have an eighth of what you leave after
(the payment of) a bequest you may have bequeathed or a debt.
and if a man or a woman leaves property to be inherited by neither parents nor
offspring, and he (or she) has a brother or a sister,
then each of them two shall have a sixth,
but if they are more than that, they shall be sharers in a third after (the
payment of) any bequest that does not harm (others). (this is) an ordinance
from allah,
and allah is knowing, forbearing."

commentary:
 inheritance of spouses from each other:

in this verse, the explanation of the heritance of spouses from each
other is stated. the verse, says:

" and you will have half of what your wives leave, if they have no child..."

but if they have a child or children, even if they are from another
husband, only one fourth of her property belongs to you. it says:

"... but if they have a child, then you shall have a fourth of what they
leave, ..."

this distribution is, of course, after paying the debts of the wife and
fulfilling her bequests upon financial affairs, as the verse says:

"... after (the payment of) any bequest they may have bequeathed
or a debt; ..."

and, if you have not any child, there will be a fourth of your
heritance for your wives. it says:

"... and they shall have a fourth of what you leave if you have no child, ..."

then, the portion of your wives will be one eighth of your property,
except for the land the explanation of which is stated in jurisprudent books.
it is in the case that you have a child, even though this child is from
another wife. it says:

"... but if you have a child, then they shall have an eighth of what you
leave...."

this division, similar to the previous division, is also done after the
fulfilment of the bequest you have bequeathed and paying the debts. the
verse, continues saying:

"... after (the payment of) a bequest you may have bequeathed or a debt. ..."

then, it states the ordinance of the inheritance of sisters and
brothers, when it says:

"... and if a man or a woman leaves property to be inherited by neither
parents nor offspring, and he (or she) has a brother or a sister, then each
of them two shall have a sixth,..."

the arabic term /kalalah/ is used for the maternal sisters and
brothers who have inheritance from the diseased person. this situation is
in the condition that there remains a maternal brother and sister from the
diseased person. but if they are more than one they will have, on the
whole, one third. that is they must divide one third of the property among
themselves.

"... but if they are more than that, they shall be sharers in a third..."

then it adds that this is in the case that the bequest be performed
formerly and the debts be separated from the property.

"... after (the payment of) any bequest. ..."

and, it is in the condition that it does not harm the inheritors
through the bequest or confessing a debt.

"... that does not harm (others). ..."

so, at the end of the verse, for an emphasis, it says:

"... (this is) an ordinance from allah, and allah is
knowing, forbearing."

that is, this is a divine order which you must respectedly observe and
he, who has enjoined you these ordinances, is cognizant of your interests
and expedients, and he is also aware of the intentions of the testators; yet,
in the meantime, he is forbearing and does not punish promptly those
who treat against his command.