What to wear

As long as you don't wear a bridesmaid dress/wedding gown, you should be fine. Chances are, you probably won't be in any of the formal photos.

I'd ask about the level of formality of the wedding, and depending on how close FOG is to his son and the bride, I might ask if they have any suggestions or insights regarding things you might want to avoid (e.g., color/style of the BMs dresses, not "competing" with whatever the MOG has chosen, etc.). I was the stepmom at a wedding. We'd been married for close to 20 years at that point, but step-daughter had lived more than 1500 miles away during our entire relationship. I wanted to avoid stepping on her mother's toes. When I heard MOB had chosen a long black and silver sequined dress for the evening wedding, I felt okay with going with a navy blue cocktail dress with some beading. It was appropriate to the occasion, but I definitely wasn't trying to "upstage" the bride's mom and it was consistent with H's wearing a black tux. Have fun!

I would ask around what the bridesmaids are wearing for colors and what the other moms are wearing and stay clear of that. A nice tea length dress is probably appropriate for a barn wedding that you can dress up or down with accessories. I think matching your date looks nice in general.