Muslim inheritance law

I am a widow with 2 children ( son and daughter). My father in law expired after my husband's death leaving behind only a son( married and having 2 daughters) and wife and orphan grandchildren of deceased son. My father in law had orally willed before his death to give orphan grand children something from the property. the proportion/share to be given was not mentioned. Fatwa was taken from Darul IFTA by my brother in law as to how much to give to the orphan children. The oral will is recognized to be fullfilled in islam, but since my father in law did not mention the share, the fatwa stated that the heirs i.e my brother in law and mother in law can decide as to what to give the orphan grandchildren. Also the fatwa suggested to give 1/3rd share to the children. Now my mother in law and brother in law does not want to give anything to the children stating the children are not the heirs in islam and does not want to fullfill the oral will. Also when my husband was alive he had given money to build house on a plot which was in my mother in laws name. Now my mother in law claims that money as gift money and not want to give anything to the grand chidren. How should I proceed in this case. Will my children get anything / how much should be claimed.