My sister is engaged to be married. She called me this morning in tears because her fiance's sister has purchased an ivory, lace covered dress to wear to the wedding. My sister's wedding gown is ivory and covered in lace. The sister of the groom was aware of this before purchasing her dress.

Should someone say something to the groom's sister? I personally think it is inappropriate as she will be in family portraits, etc with my sister in dresses that look very similar.

What the fiance's sister did is outrageous and totally inappropriate. YES, something should be said to this woman and have it explained to her just how much of a faux paus this is.

Yes, the groom needs to go to his sister and tell her, in no uncertain terms, to return the dress. It is definitely inappropriate for anyone to wear a dress similar to the bride's.

No one should ever attempt to upstage the bride. I sense a bit of yness here...