Stuck in a sticky situation

Hi, I'm a 32yo guy due to marry my fiance 29 next year who I love dearly. We have 1 child together and she has 2 children from a previous relationship who I've took on and love as my own child. The problem is me and one of my ex's have always talked and she is a Facebook friend of mind but my fiance doesn't know this. Me and my ex have very passionate sexual chemistry and slept together a lot through last year before she moved to her new home over in America. Trouble is I just want her as friend and nothing more. I don't even want her on my Facebook to be honest as when I get married they'll be wedding pics up etc. But I think my ex thinks she can always have access to me, requests I address her as a special name I used to have for her and anytime she comes down she is going to expect sex. I don't know how to go about getting her out my life as I don't know what messages she may have kept and could turn on me and tell my fiance if I don't meet her demands. So I'm treading carefully to keep on her good side. I know what I did was wrong and I regret it. But how can I tell my ex to leave me alone in a gentle way that won't entice her wrath.