My piece of advice regarding this scenario and scenarios like it: When in doubt, reverse the situation.

(I wish I could give this advice to Sabrina, as well. Did she not have a past before you?)

Different people have different thresholds for these things. I am friends with exes, and made it clear to my partner when I met him that I am friends with them. The reason I do not hang out with them alone anymore is simply because, if I reverse it in my mind, it makes me uncomfortable. I tell them honestly: "You are still my friend, but I respect my partner and in his position I'd feel like shit." This does not mean see-you-never, but it sure helps me feel I have been respectful, honest and maintained my own integrity. These are key values for me. Yours might be different.

So ask yourself: What are the values you hold for yourself? Your life? Your relationships?

Before I turn into a self-help nut, I just strongly feel that identifying these before going into and during any human relationship (romantic and otherwise) helps you curate stronger life experiences for yourself, and in my experience also makes for deeper, more trusted connections with others. If that's something you want.