Showers are often given for one group of people, not everybody who ever knew the bride, so the ex may just have wanted to(or been able to) invite one segment of friends and/or family. Anybody can throw a shower and invite whoever they want, so go to it!
And please, let your DH be upset for himself if he feels the need. Try to stay out of it, take a deep breath and go for a walk, smell some flowers, do whatever it takes to calm and distance yourself from the Mother-of-the-Bride frenzies. Your turn will come!