In many states the wife has a right to claim 1/3 of the estate, regardless of what is in the will (Called "taking against the will"). To avoid probate problems, you might consider bequeathing 1/3 to the wife and distribute the rest as you see fit, depending on how much you think the siblings deserve. (Don't forget grants to charities or other deserving folks.)
When it comes to dividing shares, it is normally an individual's choice. However, the people who are normally considered first are the legal and legitimate siblings. You can calculate the amount of shares you have and then work out the percentage for each one of them.