I've always imagined he was his disowned older brother. My generation 2 kids (Johnny's son and daughter) were best friends with Malcolm's younger twin brother and sister, who I forbid my gen 2 kids from becoming romantically involved with because it felt too much like incest.
I know this thread has been inactive for a while but I remember someone on the thread asking about what Johnny's children looked like when I mentioned I think Johnny has Landgraab genetics based on what his kids look like. My new baby challenge sim had a little boy with Johnny so I thought I'd post a photo for anyone interested (I think the little boy looks like Johnny Jr., and also a Landgraab):