You have added the 

> ** Biological child to one of the parents, not the other.

that was not in [your original question][1].

[As Ezri pointed out][2], 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][3] 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:

    Family 1
    --------
    Mother: Jane
    Father: John

    Children
    --------
    Biological, Justin, 20 May 1981
    Biological, John,   20 Jan 1987
    Adopted,    Jane,   12 Aug 1989

    Family 2
    --------
    Mother: Jane
    Father: whoever it is, or unknown

    Children
    --------
    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][4], then I'd describe it this way:

    Mother: Jane
    Father: John

    Children
    --------
    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)



  [1]: https://english.stackexchange.com/questions/36180/what-is-the-correct-word-for-step-siblings
  [2]: https://genealogy.stackexchange.com/a/2525/29
  [3]: https://genealogy.stackexchange.com/a/100/29
  [4]: https://genealogy.stackexchange.com/a/2329/29