You're probably over thinking it. Obviously if your ex is married and you're getting married there's no reason for her to logically be trying anything. I'm still friends with a couple of my exes on Facebook and will often like photos of them and their new SOs, not for shade, I'm just happy for them.
You're overthinking it. How did they end things? Are they on good terms? I like some of my ex's stuff because I like it. It doesn't mean I want them. I think when it comes to ex's, do you trust your fiance. If you do then what's the issue?