When I was in high school I got close to one of my male teachers. We had common interest and shared the same spiritual values. We talked about personal things too. I'm a very verbally affectionate person. I'd leave his room saying "Bye Mr. R. Love you!" One day he made an offhand comment about being careful how his behavior was perceived. The thought that our relationship could be misconstrued as anything inappropriate had never occurred to me. He was like a father figure to me. My dad wasn't around much at the time and Mr. R was always there to listen to me and pray with me. At the same time, he and his wife were struggling to start a family and it was his first year at that school.

I'd be interested in knowing if you husband and this woman have more of a father-daughter relationship that a potentially romantic one. What's her relationship with her father like? What's your husband's relationship with your kids? I think that it's totally possible to have a close friend of the opposite gender that's half your age. That said, both of them should be exercising full transparency. She should be open with her fiance and your husband with you. That doesn't necessarily mean you get to read all their emails or tell them they can't talk. But if you have concerns about their relationship, the amount of time that they spend in communication or the intimacy of that communication he should definitely take it seriously. 

Maybe he's not having any type of emotional affair with her, but maybe he's putting more effort into their relationship than your own. I think the best thing you could do is talk to him about it. You don't sound like you're rushing to any conclusions which is wise, but this is a concern that should be aired out now rather than months from now.