You were just acting as a protective parent before, but now that another yea's almost passed, both your son and step-daughter are that much older as well. I don't think that your sil meant to say anything wrong, but she just sees things differently from yourself. I think there's room for compromise.
Let your son go at Christmas, and try not to use the word "jealousy". Remember, that throughout his life, your son's going to like and get on with folk that you don't, but it's all about supporting your children's choices. They may differ from your own, but that's what becoming an independent young adult's all about.
You might be surprised, and find that your son actually DOES want to talk aboiut, and question some subjects now, and being the age he now is, it sometimes isn't the parents who they confide in first!
I can see why you were annoyed by what your sil said, but from her point of view, she perhaps believes that your son'd gain more confidence by tackling and talking about the more hushed subjects.
If I were you, I'd make contact with your sil, and try and have a calm and friendly chat with her, rather than see her as the villain. You could then express your reasons for feeling threatened by her remarks, and say that if your son doesn't want to discuss matters, then you'd appreciate her not pushing the subject/s again - but give her YOUR reasons for not wanting this. Sometimes, our own hidden fears can rub off on our children, and we have to learn to accept situations and to talk about them openly. Best of luck with it anyway.