Kiyonna.com has beautiful dresses. I got a cocktail dress from them a few years ago. Great materials and high quality. More on reddit.com

Dia & Co has some lovely options, largely by Kiyonna. Lace Dress - This would be pretty in sapphire, pinot noir, or, if you want to go with a summery color, coral. (I feel like September is a month where you can go summer or fall, depending on when during the month the event happens.) Second Lace Option - In addition to the shades you're trying to avoid (black and navy), this one comes in a lighter blue and the same coral as the first one. Third Lace Option - These lace dresses are all similar, but with different necklines and details. This one is really pretty, too, and comes in coral, indigo blue, plum wine, and ruby. I think the plum wine is such a pretty color if you're looking to go with a fall color. Fourth Lace Option - I'm not sure if this waist is too high for you, but I think the berry bliss color is so pretty for fall! Lace and Satin Dress - This dress in burgundy is definitely appropriate for a cocktail wedding! Embroidered Floral - In my experience, dresses similar to this one have become very popular for wedding guests. If you're willing to do black with a print/embroidery, I think this one is really pretty! Portrait Neckline - If you're comfortable wearing a strapless bra, I love this dress with a portrait neckline. Ruched Dress - I have this in black and I love the fit! I know you're not looking to wear black, but it also comes in a lovely pink. City Chic also has some great options! Strapless Ruffle - The available sizes are limited, but, if you're willing to go strapless, I think this is so pretty in garnet. Floral Print - This is the same dress as above, but in floral. It does have a black base, but the print is a large bold floral, so, if the bridesmaid dresses are solid black, it would look nothing like the bridesmaid dresses. Another Floral Option - This looks very similar to the above dresses, but I think the material is slightly different. The print is lovely! Tie Dress - This is lovely in the lighter blue! It's kind of close to navy, but, since your sister has expressed that she doesn't mind you wearing navy, I don't think it's a big deal if you wear a color that's navy-adjacent. Jumpsuit - If you want to be bold, a jumpsuit could be fun, and, as long as it's the appropriate level of formality, is appropriate for a cocktail dress code. I know jumpsuits may be different for someone your height, though. Chain Belt - It's possible I'm too into purple, but I really love the color of this one, and I think the included belt is fun, too! Reformation and BooHoo also have some lovely plus size options, though, admittedly, Reformation's sizes are more limited - up to a 3X, but I have found that their pieces run small, especially if you have a larger chest, so their 3X likely fits like a 2X in some other brands. More on reddit.com

I'd go for a solid color dress that is knee length or tea length. IME the more formal, the more a longer dress is the norm. I think you can go sleeveless or sleeved, but weather and culture play a role in that, too. One one side of my family, sleeveless at a wedding would be a total scandal. On the other, no one would bat an eye! Here are a couple examples I think would be wedding appropriate, from one of my favorite stores: https://www.unique-vintage.com/products/hell-bunny-plus-size-1950s-red-helen-swing-dress https://www.unique-vintage.com/products/collectif-plus-size-1960s-black-mesh-arionna-wiggle-dress https://www.unique-vintage.com/products/unique-vintage-plus-size-black-knit-ruffle-sophia-wiggle-dress https://www.unique-vintage.com/products/unique-vintage-plus-size-black-swiss-dotted-mesh-norma-jean-wiggle-dress https://www.unique-vintage.com/products/unique-vintage-plus-size-1950s-black-sweetheart-midge-swing-dress More on reddit.com

Unique Vintage has some white, tea length dresses. It looks like you can pay for faster shipping too More on reddit.com