By all means, I wouldn't condone being frosty to the SIL or the step-nephew. Be gracious and welcoming. Try to get to know her and the stepson as much as you can.
But for the thoughts that I had regarding my personal take from my own relative, I would entertain the possibility that gracious and welcoming might not be enough for this brother who would get married in secret and then announce it to a close family member via Facebook (when he certainly had the opportunity to come clean when he asked OP to set two more places for dinner). I could imagine my relative, in this case, asking several times throughout the course of the dinner things like, "Were you surprised?", "What do you think of the news?", "What did you think when you read it on FB?". And each question would be asked ad nauseum. And any attempt to redirect the conversation would be viewed as being rude and unsupportive. It's all meant to create drama.
Now, I'm not saying that OP's brother is like this, but I would be prepared for this. Having lived through many situations like this, it's where my mind goes. And forewarned is forearmed.
(Said relative of mine was once in a car accident and was unconscious for several days, during which we didn't know what was going on or when she might wake up. No explanation, transferred to a major hospital...etc. So when relative finally woke up, she didn't really ask much about the accident, or how her kids were taking it, or anything like that. She kept asking if we notified this friend or that friend that she was unconscious, that she'd been intubated. She was more concerned with the drama and the attention that she got from being unconscious and intubated than with anything else about her accident or her health. And this was par for the course with her.)