I don't know if I'm going to be of any help here...

I get the impression that your sister in law is just starting school, in which case, there are certain courses that have to be taken regardless, as you know. Anyway, she has plenty of time to change her mind many times, and she probably will. I seen nothing wrong with telling her that she would be a great counselor IF she comes to you for advice or the conversation goes that way.

As far as you keeping your past to yourself...I would. If your MIL finds out, then there will be judgments, discussion, and comparison to your MIL's sister. You will be gossip fodder. It's your own business, keep it that way.

If she denigrates counseling as a profession in front of you again, I would just make clear that you think it's an honorable, fulfilling career. End of her comments (maybe..)

Just one final comment. I had my heart set on teaching. I had been told by many people that I was born to it.
My parents told me that it was a very difficult job and I wouldn't be good at it. Sadly, I listened to them because, at that age, I think we're still influenced by them. If someone had explained to me that they are flawed individuals who don't always have the right answers, I would have stuck with it. I understand that I was too heavily influenced at the time, but the past is past and the damage is done.

Good luck to you and your sister in law.