If the couple didn't give any indication of dress code, it's awkward. Sometimes you can get an idea from the venue: My cousin's reception was in the ballroom of a swanky hotel, so I knew it would be dressier than my nephew's, in a tent on a farm.
I think a glam version of your usual style, with velvet leggings (or some other fancy material, maybe faux leather), cute ankle boots, and a fancy top, could work for a lot of dress codes. Here are some ideas that might feel like "you". Also, Ms Maven posted in the forum recently about a couple kinds of velvet leggings.
You can think about what the couple likes too; if they've complimented you on what you're wearing a couple times, maybe you can do a "festive" version of what you had on then.
Let us know what you think, and post a few pictures when you can in a couple days.