Grandfather's property:-
As per his last will, the properties in his name should go to his son and grandsons(male heirs of his son) only.
No. of son=1 and No. Of grandson=4. Therefore, total claimant = 1+4=5.
Therefore, the properties in your grandfather's name should be divided by 5. Each (one son and 4 grandsons) will get 1/5th share from the whole property.
If your father has died intestate leaving behind your mother and his four sons. Then the 1/5th share of your father will be divided into 5 parts (one part of share to mother, one part each to 4 sons).
If Elder brother (his share is = 1/5th of grandfather's property + 1/5th of father's share of grandfather's property) died intestate leaving behind his wife, a daughter and a son, then Elder brother's share will be divided by 4 (because, besides, wife, daughter and son, mother will get 1/4th of the property of Elder brother)
Your mother can make will for her part of shares only which she got from father's share and Elder brother's share of grandfather's property.