I read more post here than I care to admit for insight and none really address this.
I had the privilege helping raise my SD after her mother passed away. I love my stepdaughter as my own, but she did have a mom who loved her very much. Weddings drag up memories and regardless of my position (SM), I want to share in her excitement and festivities without overstepping etiquette, tradition or adding stress and pressure to her. In the end, I asked my SD if I could give her a party, not a shower just a party to celebrate her. Mind you this would be a party that would include many that were a big part of her life (step-aunties, family friends and step grandma) but most wouldn't be considered for the engagement party, showers or wedding. We all still want to celebrate her and shower her with love. I felt this party was a good compromised and certainly a honor. I picked a date 6 months from the festivities and after the engagement party, so the news was already out, and it was far enough away as to not be confused with a shower. When considering stepparents, step aunties and family friends, this is just an idea to be involve them. In my day churches would give such a shower for the bride. The hard part is to figure out whom you'd ask to attend. While our story is a little different it just a thought....