Distributing an estate according to an unislamic will

My mother passed away not long ago (my father died 20 years ago) and she left a will that her home was to be sold and the proceeds shared equally amongst us 4 siblings - 2 brothers and 2 sisters. However, one of my brothers said he is entitled to twice the share of us sisters (other brother agrees to equal shares). I asked my local imam and he said that if a will was not made then Islamic law would be put into effect but my mum was in sound mind when she made the will and there is no problem with sharing it equally amongst us. Is this correct?