My fiance and I are having a formal wedding in the sense that we (us and the wedding party) are wearing fancy garb because we love dressing up. After some of the bridesmaids let it slip that they were wearing long gowns, a lot of guests have let us know how excited they are to finally "wear that gown in the back of the closet" or "dress up a la Mad Men." We really don't care what people wear as long as they are ready to party and don't show up naked
The problem lies with the Mothers. My mom is busting out with a custom made gown. She thinks it would be thoughtless of me to not tell Fiance's mom and stepmom to warn them that people are dressing up and that they should, too. I think it'd be rude of me to tell them what to wear if they don't ask me and that they should wear what they want. They aren't the type to dress up in more than a nice cardigan and slacks and I wouldn't want to put a financial burden on them by having to get new dresses. But I also don't want them to feel uncomfortable or out of place.
So would it be thoughtful or rude if I gave them a heads up on what most people are wearing?