She died and left behind a mother, two sisters and a husband

The mother gets one-sixth, because of the presence of the two
sisters. Allaah says (interpretation of the meaning):

“if the deceased left brothers or (sisters), the mother
has a sixth”

[al-Nisa’ 4:11]. 

The two sisters get two-thirds, because of the words of
Allaah (interpretation of the meaning):

“If there are two sisters, they shall have two-thirds of
the inheritance”

[al-Nisa’ 4:176]. 

You, the husband, get half, because Allaah says
(interpretation of the meaning):

“In that which your
wives leave, your share is a half if they have no child”

[al-Nisa’ 4:12]. 

The sum of all these shares is greater than the estate, and
the scholars call such cases al-‘awl, i.e., the sum of allotted
shares is greater than the estate. So the share of each of the heirs is
reduced in line with this excess, which in this case is one-quarter. 

In that case the estate should be divided as follows: it
should be divided into eight equal parts. The mother gets one part, the
sisters get four parts and the husband gets three. 

Each heir is given his or her share as follows: 

The mother’s share = the estate x 1 / 8 

Each sister’s share = the estate x 2 / 8

The husband’s share = the estate x 3 / 8

And Allaah knows best.