Attire For Girlfriend Of Father Of The Groom

I`ve been dating the father of the groom for the past 10 years. (no, ladies, I am NOT interested in remarrying) His oldest son is getting married in October and I am stressing over the length of my dress. The grooms parents are divorced and mom has remarried but dad has not. The bride is insisting the groom`s dad does not need to wear a tux (the stepdad is wearing one) and a suit will do just fine. The wedding is "black tie optional". I thought a long dress - not as dresssy as mother of the bride - simple but long, satin, was appropriate but I was advised that I was "just" the girlfriend and should not wear long. What`s proper. I do not want to stand out like a sore thumb or be under dressed but I also don`t want to overdress. Any suggestions?