Dress for her?

my elder sister, who is married just came back from Canada for few weeks. we have a plan to go to a resort next week where a lot of activties including musical concert etc are gonaa taking place. I have bought few pakistani dresses for her from boteeque. unfortunatly i could not get the right sizes and at least 2 are short in lenth and provocative as well as they are too tight. I'm nervous as many people thr would be knowing us and it will be a embaring situation. I want those dresses change but she feels comfortablw with them and i,m finding no excuse how to sk her ples let me change this stuff.how to talk to her?