Wow she sounds very selfish, I wish I had some advice but fortunately I'm as close to my FMIL as I am to my own mother. The white dress to the wedding would definitely throw me over the top! Go ahead let her wear black. Just try not to let this woman ruin your day, you will probably never get a long with her but you love her son and even though she lives close by you'll just have to stand your ground, but at least your FI is pretty good about standing up to her on your behalf I think that's something that is rare in these situations.