I want my fiance to wear a family wedding dress instead of my sister
Why do you want to choose your fiance over your sister?
I will be more happy to see her wear the family wedding dress instead of my sister.
You should be considerate of your family's feelings when making demands. Try having a calm discussion with your sister if you feel strongly about this topic. She may feel angry if you go behind her back and take the dress for your fiance without her input.
The wedding is about me and how I feel! It is my day after all.
But your sister feels the same way about her wedding, and possibly even more so since she will be the bride, so consider her feelings. She might be okay with your fiance wearing the dress as long as she gets to use it as well. See if that is an option.