Wait... what? I was seriously expecting a "just kidding" at the bottom of your post.
She WHAT? And your FI did NOTHING?
First, you need to have a heart to heart with him. He needs to squash this behavior now or your FMIL is going to think she can run every detail of your life. Right down to the kind of dish soap you use and which hand you wear your watch on. This needs to stop, it needs to stop now and your FI has to be the one to tell her to back off.
With regard to your FBIL's gf, unless you really want her as part of your wedding party, have a heart to heart with her. Tell her that you care for her, that you enjoy spending time with her but that your heart was set on having your very dearest and oldest friends/sister/whomever as your wedding party and that your FMIL was out of line setting such expectations. To make your point, remind FBIL's gf that if it had been a sincere request, don't you think you would have made it yourself. Then give her some other task - guest book, usher, program distributor... whatever. As for the dress, again, tell her that she is welcome to wear it, but that if it isn't just what she wants, she shouldn't feel obliged.
But get your FI to be a man and stand up to his mommy - or you will suffer FOREVER. Believe me... I know aaaaaaallllll about this one.