Oh, now you've roped me back in.
This is what was explained in a link I posted much earlier today, that went to a very interesting thread over on courttv!
I'll try to explain it as I recall it. JP apparently (according to a poster on courttv who got it from vital records in San Diego) had 2 children while in her late teens, one at 18 and one at 19 and gave them both up for adoption.
The one you mention is one of them, I believe. It's a pretty convoluted family make-up.
After those 2 children, she then married the man who was to become SP's father, he already had I suppose 3 children, and then they had 2 more between them, one of them SP.
Here, go to this link, and read the things posted by celestrill. Explains a few things about them, I think.
Apparently after Jackie and Lee married John took Peterson as his last name, no official confirmation of adoption found per RTE66.
Scott was born the year after Jackie and Lee married. John was born in 1966, Scott in 1972.