I think that EVERYBODY knows Sue Ellen's sister Kristen was Christopher's biological mother. Everyone ALSO knows that Sue Ellen HATED Kristen - and was indifferent at best when she was murdered. She was also indifferent to Kristen's son - or she would have petitioned the courts to adopt him herself. After all, SHE was Christopher's next-of-kin.
But Bobby has been such a great father to Christopher that most people don't even remember - or care - that Christopher isn't his biological son. Christopher certainly doesn't care. As far as he's concerned, Bobby Ewing is his father, period. He even considers PAM his mother - even though she hasn't been around for YEARS.
And Bobby NEVER talks about the biological son he had with Jenna Wade. With Christopher in his life, he has no need for another son. And when he said that the day he adopted Christopher was the "...happiest day of my life" - I couldn't help it. The tears - they were a' flowing. It is truly an enviable father/son relationship - much better than the relationship shared by J.R. and John Ross.
I think that's why John Ross is so hard on Christopher: he's jealous. And it doesn't matter that he and Christopher are related by blood - John Ross doesn't care about his MOTHER'S side of the family. Only the EWING side - which is why he always mentions that Christopher isn't a Ewing.
Hope this helps.