Rephil: You already are a "true" father. For your other two children, your wife, and you, too...try not to make too much out of the birth of this child. Your son and daughter will, in short order, perceive that this child means more to you than they do. Don't let this happen. It can be devastating to some children and to a marriage.
My extended family is made up of many blended families and adoptees. We learned long ago that most anyone can procreate. It takes loving parents to raise children.
Congratulations to your son, daughter, wife, and you.