You have added the
** Biological child to one of the parents, not the other.
that was not in your original question.
As Ezri pointed out, that makes your word "step" completely wrong. Doug would be a half-brother to Justin, John and Jane, and not a stepbrother. However, there is no such thing as a "half-son". Doug is a son to one parent, and a stepson to the other.
The way most genealogy software works, and the way you would normally represent the situation you describe, Doug would not be considered part of this family. Doug would be part of the family of either:
John and John's first partner, or
Jane and Jane's first partner
Assume it's Jane and Jane's first partner. Then what you would have is:
Biological, Justin, 20 May 1981
Biological, John, 20 Jan 1987
Adopted, Jane, 12 Aug 1989
Father: whoever it is, or unknown
Biological, Doug, 1 Dec 1979
Notice I also changed the date format. You should NEVER use mm/dd/yyyy which can easily be mixed up with dd/mm/yyyy.
Normally "biological" is assumed and need not be stated. I prefer to state "child" rather than biological, and advancing that further prefer "son" or "daughter" over "child".
If you still wanted to list the four children together as a non-traditional family, because they lived together for some period of time, then I'd describe it this way:
Son 1, Justin, 20 May 1981
Son 2, John, 20 Jan 1987
Adopted daughter, Jane, 12 Aug 1989
Son of John and John's first partner or
Son of Jane and Jane's first partner, Doug, 1 Dec 1979
As you can see above, there is no simple way of describing non-traditional family members as part of the family. You could also say:
Son of John but not Jane (or Jane but not John), or
Son of only John (or only Jane)