Another Mormon website is http://www.clothedmuch.com/
Elaine is such a cutie, and has a lot of fun with her clothes while still keeping modest. I don't think she has anything on wedding receptions, which are held in a hall or a church meeting hall. You don't have to worry about appropriate temple attire--only members in good standing can attend the actual temple ceremony. I have Mormon relatives.
You don't have to wear long sleeve. Short sleeves, cap sleeves or even sleeveless will be okay.
All the advice above is good. If your dress/outfit is close to the knee, not too tight, and doesn't show too much skin on top you will be fine. Cleavage will draw stares. I see you are in the mid-west, and if the reception is held there, you will probably see a wider variety of clothing choices.
And yes, the teenagers tend to be far more adventurous (shall we say) than the older folks.