Division of the estate of a man who died and left behind a wife, mother, brother and sister

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.