I'm meeting my boyfriends fam on the 4th of July? Please Help!?

I'm meeting my boyfriends extended family on the 4th of July. What should I wear? I don't want to look too dressy, or too casual. I definitely don't want to look "slutty", or anything. But, I do wear a size DD and it's hard not to have them out.. I don't know what to wear! It will be all his family, 2 aunts, an Uncle, 3 to 6 cousins and his Dad and Step mom. I've met his parents before, just not everyone. Please Help.