Being fair to her fiance is fine and all l, but this advice is supposed to be in your interest. If( only if) you do want to pursue her I think you should do something that will make it clear to her. Leaving her with no choice, but either reciprocate your feelings or reject them. If she acts ambivalous then you should distance yourself letting her knows you aren't interested in being some side guy. If she does reciprocate as not be evil about it pressure her to make decision whether to get with you and leave her fiance or if she wants to stay with him than while also meddling with you then you need to run. In this case she is a hoe.

If she just rejects your feelings than disrance yourself from her. Things will be awkward, but there is no potential reward without potential risk.