This is a tough spot, but I do believe that when a woman is taken she can still be sexy, but in a more understated way out of respect for her relationship, otherwise she is giving off the signal that she is still available. Tell her that she is beautiful but you really want her to save the more sexy stuff fro you alone--then take her shopping and buy her something cute and flirty that isn't revealing to give her an idea of how you would like to see her dressed--you c an win any woman's heart by taking her shopping.
Also, do it in a way that makes her feel you love her body and want to see thing that celebrate it without being too out there.
Find a way to tell her that she has a great body and that she should show it off, but that you get a little uneasy when she dresses so revealing that other guys start to think that she's trying to pick somebody up. If she's really into you, she'll understand and tone it down a bit. Or tell you why she disagrees.