if you have a very trusting relationship, and neither of you has cheated in the past, then i'm sure it will be ok. i can understand why you feel threatened, i'm sure all women have felt the same at some point. it can be difficult because as you aren't one of the managers, there will be a lot of private meetings, and a they will be spending a lot of time together. if she is flirting more with your partner than anyone else, make it clear to her how serious you both are about each other. the silly cow probably just needs a good sh*g... well she'll have to get it elsewhere wont she?!