It could very well be something the counselor suggested.
It could be that he is being encouraged to think about how many lives one marriage affects.
If he is sincere, embrace him and encourage it.
Supporting him in this new catharsis is being supportive of your sister.
It sounds to me like he is making huge headway.
It takes a lot of courage to admit your shortcomings publicly and apologize, much less to your in-laws.
Read the book, if you haven't already. Next timethrow in a few things you gleaned. I know when I've heard a great sermon or been to a great class, I'm excited to share what I learned. When people look back at me like ----uh huh, ok, whatever - well, that's embarassing and hurtful. The new thing I was so excited to share isn't the thing getting rejected, it's me. But when they are happy for me and interested and want to know more themselves, it affirms and encourages me and lets me know I'm on the right path and not a freak.
I'm afraid you guys are making him feel akward for trying and this in turn, will only hurt your sister.