Praise be to Allaah.
The man's estate should be divided among the heirs mentioned in the following manner:
The wife is given a quarter, the mother is given one-sixth, and the rest is shared between the brother and sister giving the male twice the share of the female.
So if, for example, he left behind 18,000:
His wife gets one quarter of the estate, which is 4500.
His mother gets one-sixth of the estate, which is 3000.
The rest (18,000 – 7500= 10,500) is divided into three shares.
One third is given to the female, 3500.
Two-thirds are given to the male, 7000.
And Allaah knows best.