How do I relate to my (22F) sister (18F) after my fianc? (23M) fell in love with her?

I have always been extremely close with my younger sister. We've been each others' closest friends and have always shared everything, the good and the bad. We would stay up late on nights over weekends talking about everything under the sun.
One thing to know about my sister is that she is naturally stunningly beautiful. Like, when she walks into a room all heads turn, male and female alike. She doesn't do it on purpose, and she doesn't even think about her beauty or accentuating it at all. She's never had a boyfriend and is remarkably innocent about the way things work between genders.
I got engaged in January of this year and, very long story short, my fianc? fell in love with my sister once he met her. He "didn't mean to", whatever that means, but I broke the engagement, in early May. I moved out and have been working through my anger, grief, and depression, and it is slowly going away and I am finding myself enjoying spending time with family and friends again.
The issue is my sister. Every time I see her now all I can think is "that's the beauty that made my fianc? choose her over me." I desperately don't want to be thinking about that - she had no part in it, apart from being (imo) too overly friendly to him - but that's where my mind goes, and I am having a hard time being close with her. I really want to be able to go back to the kind of closeness we once had, but I clearly have something unresolved that's blocking it.
How can I stop thinking about my sister as the "other woman"? I haven't told her what exactly happened, because she'll be devastated, but it's like I close down every time we talk.
Tl;dr - I'm having a hard time being close to my sister after my fianc? fell in love with her.


Source.