Groom's Sister included as a "Groomsman"

I'd let her wear a dress similar to the bridesmaids in grey

I originally wanted her in a grey dress in the same style as the bridesmaids. But my fiance didn't want this. So I am trying to compromise with the pencil skirt and shirt, but I really am concerned about the tie.

Only go for the "funny and cute" thing unless she is very comfortable with it. We wouldn't do the opposite thing to a guy on the brides side (put him in a dress to match) so I'd be uncomfortable having a girl dress like a guy to stand on the grooms side

This is your future SIL-- what does *she* like? In the end, this your FH's side, give him a lot of lee-way on this. If she's comfortable, why worry if she looks too masculine?

I have been determined not to make the wedding all about me. After all, it is both of us getting married. That's why I was letting him choose what he wanted for his sister to wear. She just tells us she will wear whatever.

JC- I really like that idea. I could use the color of the ties and it would look a little more feminine than a tie.

Zoe- You are right. I am blessed to love my soon to be inlaws and don't want to make her uncomfortable in any way. She has made comments previously hinting at she may be uncomfortable with certain things (like escorting bridesmaids). I just want to make sure she is comfortable and feels beautiful that day.

My vote would be to have her wear a gray dress in the same style as the bridesmaids are wearing, but if that is not what she and your FH want to do, I would let them decide. She is playing a unique role so she really doesn't have to match anyone. It sounds as though she will be coordinated with the groomsmen color wise, so don't stress and let her pick what she (and FH) want.