Sir, I presume the family is a Hindu family.
1. In the absence of a will by the deceased, both his self earned property and the ancestral property should be distributed equally among his wife, son and daughters.
So, his wife can, by will give away her share to anybody she wishes to. This will is valid one even before partition.
Here, the legal heirs of the deceased are his wife, son, and daughters. So, each one will have their share in the property by default.
After A's wife's demise, her property will be divided among her legal heirs, i.e, her son and daughters in the absence of her husband.
2. In this scenario, a son can have property in his name only if father has made a will to that effect of his self earned property. In father's self earned property, his wife, son and daughters will have a share and so, cannot be given through a will.
If the son gets property through will, then his mother and sisters will not have any share in that property. But his wife, and daughter will have their share.