Dress or Pantsuit?

Duckie&Cricket

My FI wants to include his "sister" in the WP and one of the GM (or GW as the case seems to be). She is his best friends sister, but they are very close.I am totally OK with her being on his side in the WP, since I hardly know her and have my own besties, it seems a nice way to include her. I am just trying to decide if it would 'look' better is she were in a dress or a pant suit. Had anyone been to a wedding where the Groom had a woman in his party? What are your opinions on the clothing options? 

(BTW, it would be a very feminine suit. I am not going to stick the gal in a Tux and try to make her look like one of the guys.)

Duckie&Cricket

To update the situation, she said she would wear either. Basically whatever I told her to wear. But the FI and I decided to put her in the same dress as the BM but in black with a sash to match the GM. She isn't much of a dress kinda gal, so I wanted to give her a more comfortable option with the pants. The FI liked the dress idea better so we went with that and she seems to be liking it so far.

The reason she is on his side and not mine is because I have only met her once and she is close to him not me. That is what he wanted and she is ok with it. I honestly don't care if people think it seems weird to have a girl on the "guys" side and she doesn't seem to me to be the kind of person that is going to worry too much about it. Plus, most people there will know who she is and that she is practically his sister so they will understand why she is with him and not me.