What is the effect of having one brother and one sister, or three sisters, on the mother's share?

Primary text

If the heirs include one brother and one sister, or three sisters, alongside the wife and grandfather, they reduce the mother's share from one-third to one-sixth (1/6). The remainder is then divided among the brother(s) and sister(s) based on a ratio of five parts. The distribution is sound if calculated from sixty.