is this *just how it's done*, or is my SIL being over-controlling?

[Minmom3]

White Lotus - have you considered finding a seamstress and having a dress made for you?  There are plenty of patterns that would work.  The seamstress can take an existing pattern and alter it enough that it REALLY fits you, and is flattering.

My DD#3 is starting a business of making custom dresses for people.  She's starting small, with her buddies, and she meets with them and drafts a basic pattern, and then can adjust it to be different for successive dresses, depending on the desired final item. A person who has not been through a clothing design and construction course, or pattern drafting course, should still be able to take an existing pattern you bring in and make sure it fits properly.  Call your local fabric store and ask if they have a list of sewers they hand out.  Yelp any names and see what comes up.  Check Craigslist and see if there's anybody local to you.

I had my wedding gown - a period skirt and blouse - made by a local seamstress, as it was well over my sewing skills and I didn't want to destroy and waste my beautiful fabric.  I don't remember how we found her, but I know we found her for this purpose, we didn't know of her before that.

[cicero]

I am looking for a new dressy/cocktail type dress.  Our Temple dress code is like yours, and I need to be able to wear this in the Temple, and to black tie evening things, like receptions, which may follow.  Also, as a rapidly aging lady, I do personally want long sleeves (not sleeveless/strapless and a "wrap."), a non-skin-tight fit, and a knee-length or longer skirt.  I can't find anything!  Not even in large and expensive department stores.  I have lately been looking on "modest clothing" sites, which seem mostly LDS (and thank you for them, LDSers).  If you can rent something suitable where the wedding is being held, go for it.  You may find it hard to get anything suitable in the US.  I would call someone to ask about the colors, and there is nothing at all wrong with doing so.

White lotus - have you tried in orthodox jewish areas? you can usually find the long sleeves, longer skirts, etc.