Just wanted to give you props for working "My husband is a doctor" and "I own a Cadillac CTS" into one post.
Seriously, I don't think your SIL is trying to insult you or your husband. He's her big brother. She probably doesn't think of him as a doctor just as a brother. And an invitation to a nephew's graduation isn't as formal as a wedding invitation, so I wouldn't be too upset.
I didn't need to actually. I had a separate post previously for my new car. You must have missed it!

Back to the topic. If she wanted to drift way from formality, why Mr. and Mrs. That is considered formal. Why not first and last names and no title? And no, I am not "upset". I am curious.