My stepson expressed unhappiness at the thought of his half sister being born on his bday..help!?

Here is one solution possibly, when it comes to Christmas and gifts from you and your husband, find a way to mark the presents (no gift tags) so your step-son doesn't know who is getting how many gifts. Explain to him what and why his Grandma gives more to him and his sister (especially if it's the grandma on his mothers side). It may help to read him something from a book that states a baby comes when it's ready and if by chance the baby comes on his birth date then he'll have a birthday party after you get home (or before) and the following year he'll have the option of having his party first or after his sister.