I think the obvious difference between SG and the couple you're dating is that SG is single. It makes sense to me that you don't feel as threatened by the couple because you know they have each other, and you're in relationship with both of them, while SG is something of a wild card. There might be a part of you wondering if she wants your husband all to herself, even though there hasn't been evidence of that. A fear of losing your husband could twist in your gut, as you say.
You didn't mention whether or not you've actually met SG or not. I agree that doing so would be a good step, if you haven't already. In fact, she should probably meet the other couple as well, since your husband is involved with the woman there, too. It might not turn out that SG socializes with all of you or develops a friendship with you, but knowing that she has met you all in the flesh might help you. She will know all of you are real and that she is but one part of your husband's life.