Making up an Islamic Will

My father in law is very sick, and he wants to retire from work, he wants to make up his will now because he will need major surgery soon, he wants to give my mother in law 50% and my husband 50% of his provident fund and of his house. He had three sons, two of his elder sons passed away, and my husband is the only surviving child. Both deceased sons have a 1 daughter each, they both married non-Muslim girls and when they passed away their wives went back to their religion and also changed the children. My in laws have no access to the children, and only visit them when their mothers allow them too. My question is, do these children also get a share in my father in laws will as their fathers were also heirs. Please advise what is the sharia compliant share for all these heirs, as we do not want him to be answerable for making an incorrect Islamic will.