Attire for sister of the groom attending as a guest

My future sister-in law opted out of being a bridesmaid. She recently posted a picture online and stated that she was thinking about getting a dress very close in color and similar in cut to the bridesmaid’s dresses. When asked about it she told my fiancé that we can’t control what guests wear to the wedding.

Is it ok for us to ask her not to wear a dress that similar in color etc to the bridesmaids? Though she is not in the wedding party we plan on having her take part in the wedding mass in some way. My fiancé and I both thought it would have been appropriate for her to ask us if her attire would be ok since she is so closely related and will be involved. We assumed like our mothers she would want to choose a complementary color rather than a similar color to the bridal party.

Are we over reacting? Is it appropriate for us to ask her to wear a different color?

Donna, Wedding Queen, President; Top Wedding Sites, Inc®, a wedding planning guide, and Recent Mother of the Groom –

Guests shouldn’t wear colors or styles that resemble that of the bridal party, but you can’t really control what guests wear. They should know what’s acceptable or try to find out.

Maybe her brother should speak to her about what’s proper. She might feel less embarrassed if she was directed in private.

Rebecca Black, Etiquette By Rebecca

I agree. If the dress is truly similar to the bridesmaids dresses, her brother should speak to her privately. If the dress is just similar in color, then it shouldn’t be too much of an issue. But, she really didn’t need to consult anyone about her choice in attire. It would have been a nice gesture though.