Hi, M. -- I'm doing the math here - You mention that she is six and a half but say she's in kindergarten. That would make her a good bit older than most other kindergarteners, if I have that right. Was she held back on purpose for a reason? She might be frustrated with being the oldest and biggest kid in her class and showing you that she can get her way. She might also be, as the other poster rightly said, adjusting to the newness of kindergarten and acting out to get attention.
It draws negative attention from you and her dad and your fiance, BUT negative attention is still attention-- and kids often don't care if it's negative (discipline, time outs etc.) as long as they're getting some form of attention.
Perhaps more importantly, not only is there the new world of kindergarten for her to navigate but remember: Her mom is engaged. That means she will not be getting her old mom-and-dad life back, and things will change yet again for her. It's great for you but see it from her perspective -- possibly she is "especially sassy," as you say, towards your fiance because she is understandably jealous of him. You can't blame her if she is, no matter how great a guy he is. To her, he's spending time with you she may want for herself, or maybe she sees him as trying to replace her dad. She's more aware than you may know that the grown-ups are changing her world for her and she has no control at all over that. This does not justify her behavior! But it can help explain it.
Make extra time for her. Spend time alone with her without your fiance. Catch her being good and praise her a lot even if you feel like you're praising her for things she should just do anyway at her age (like just being polite all day). Reassure her that she is still important and isn't being replaced. Get her dad to be less "scary" if you can and more reassuring that he too will be in her life.
Of course you can't let the sassy events go unnoticed -- she does need to have consequences but ones that happen immediately and mean something to her. Rather than time outs try taking away something that matters to her like TV time or a favorite toy etc., and announce/do it the moment she is sassy, but stay calm, calm, calm, not raising your voice etc. She back-talks, then she gets, "Because you said XX in a way you know is completely unacceptable, you have lost your XX until day after tomorrow. You made the choice to do XX and this is the consequence." No negotiations but no yelling either. Walk away. Combining swift but calm responses with more attention and more praise could help her through what may be tougher times for her than she's letting on. Good luck.