1. The Owner of the property can only dispose the property in any manner he likes.
2. Since your deceased father was the absolute owner of the property, his wish is to be carried out after his death by following the contents of the WILL written by him, if the WILL is attested by two witnesses. Then the property would devolve to your mother & thereafter her demise, the property would devolve equally to all her sons.
3. Assuming that the property still stands in the name of your deceased father, your mother is not the owner of the property at the moment and she cannot execute a WILL bequeathing the property only to her 2 sons superceeding the WILL executed by your deceased father.
4. All the 7 of you can enter into a family settlement deed regarding this property and divide the property amongst yourselves in the ratio to be decided amicably by all of you and settle the matter once and for all.