Late September Evening Wedding-What to Wear?

This might seem like too early to ask the question, but it takes me forever to find dresses that fit, soo here we go.

My boyfriend's brother is getting married on September 29, and I'm going with him. It's going to be an evening wedding (around 5) followed by a reception. I don't know what to wear to it, as it's going to be very formal and very elegant. I'm not skiny (size 12), but I do like wearing dresses, and I'm assuming that would be something good to wear to this. I just don't know what colors or styles to wear. I'm not sure where it is being held, but I think the ceremony is in a church.

I don't want anything that will offend my boyfriend's relatives (especially since I'll be sitting with his parents and his other brother's girlfriend since both brothers are in the wedding party), but I also want to look good. Any ideas?

Late September Evening Wedding-What to Wear?theatre

Try to wear a conservative dress, nothing too tight, to short, too bright, no white, no plunging necklines.

A midcalf dress in a neutral color would do. Pink, blue, burgundy, emerald are good choices for a fall evening wedding.

Stay away from halter dresses or sweetheat cuts that may accentuate your bust too much. May sure to have a shrug, shawl or a cardigan handy during the cermony.

Late September Evening Wedding-What to Wear?opera music opera theater

A classic cocktail dress.
you can wear my trousers
If it's a formal evening reception, wear something you would wear to a cocktail party or slightly more formal. Avoid solid black or white (black still has connections to mourning in many people's minds and can be seen as insulting in some places, while white is the bride's privelige) or anything flashy enough to upstage the wedding party. If the wedding is being held in a church, keep your shoulders covered (a small jacket or shawl is great if the dress you choose has bare shoulders or is strapless) out of respect.

Other than that, find something you look and feel great in, and party away!
A solid, fall color dress with some shimmer to it would be nice. Try to stick to red, orange or brown, since those are the colors of the season.
A cocktail style dress is appropriate for that time of day. If you know what the bridal party colors are try to avoid it. The dress shouldn't be something you could wear to work but it doesn't have to be a ball gown either. (not unless they specificed Black Tie on the invitation). If the ceremony is going to be in a church try to be conservative with your top (strapless or otherwise) or carry a wrap with you so you can cover up if necessary. It can always be removed for the reception. Good Luck!
Size 12 is a good size, to begin with. Now seeming that it will be fall, depending on your skin tone and hair color, Choose a color that suits you. Example, if you are fair, a nice burgundy or similar class of color. If you are dark, anything in the middle tones, such as light tangerine. Medium skin tone can get away with just about anything. Now as for style, just keep it simple, but elegant. The best of luck shopping, you'll have fun trying on all the variations.