Is your dress a traditional style wedding dress? That's obviously a wedding dress? If so, I really don't think there's anything to be concerned about.
If your dress is non traditional and could be worn to a different occasion, I can see why you might question it, but unless MIL knows what you are wearing she can't be acused of being inappropriate.
My mil and grandmother in law both wore lilac, which was the colour the bridesmaids wore. I could not have given a crap and the photos were lovely.